Spectroscopic Evaluation of Colour Grading of Type Ia Diamonds†.

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Rajendra Ardalkar, Sandesh Mane, Anik Goswami, Mahesh Gaonkar, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4593034/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Colour grading of diamonds is determined routinely by experienced graders, and grades range from D to Z, with D being pure white and valuable. Efforts by researchers are focused to develop an instrumental method for bias-free grading. Presence of nitrogen in type Ia diamonds results in yellow tint and its intensity increases with increase in nitrogen content (grades from D to Z). In the present work electronic and vibrational spectra of diamond standards of Gemological Institute of America (GIA) were measured to develop and standardize the methodology by utilizing nitrogen based vacancies. Data on platelet peak position and its width were analysed for a possible correlation with the colour grade. Platelet peak position data obtained from IR spectra gave a good correlation with colour grade upto grade L. For the grades E to M, data fitted well to both quadratic (R 2 = 0.92 ± 0.02) and linear equations (R 2 = 0.91 ± 0.01). Using peak position fit, colour grades of 336 diamonds were evaluated and compared with visual grades that were determined by the graders of Gemmological Institute of India (GII) and results are discussed. Absorbance of N3 Peak (415 nm) in the visible region also gave a good correlation with colour grading. Colour grades of diamonds referred as test samples were determined using UV data and the results are encouraging. Therefore, it appears feasible to develop an instrumental methodology for colour grading of type-Ia diamonds based on only absorption measurements in the visible region. Vacancy in diamonds Platelets frequency IR spectrum UV-Vis-NIR spectrum N3 centre Nitrogen Aggregate Full Width at Half Maximum Fitting of data Colour grading of diamonds. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction Natural diamonds are evaluated to enhance the scientific understanding of their diverse colour variations, unusual inclusions and cutting flaws associated with them. Natural diamonds are highly sought after for their brilliance and commercial value. Their value is determined by well-known 4Cs namely Colour, Clarity, Cut and Carat weight (Bouman et al., 2018 ; Breeding & Eaton-Magaña, 2019 ; Eaton-Magana et al., 2020 ). White colour diamonds are the best grade which is denoted by grade D. Other diamonds are graded lower than D (Eaton-Magaña et al. 2018 , 2019 ; Breeding et al. 2020 ). In the past, various grading systems were used to denote the colour grading of diamonds. At present, GIA (Gemmological Institute of America) grading system as well as CIBJO (Confédération Internationale de la Bijouterie, Joaillerie, Orfèvrerie) grading system are being followed internationally (Eaton-Magaña et al. 2018 , 2019 ). In both the systems, D (white) represents the best grade diamonds and Z represents the lowest grade diamonds (Eaton-Magana et al. 2019 ). Colour grade scale for smaller diamonds (≤ 0.15 carats) is divided into broad groups by GIA (King J.M. et al. 2005; Breeding and Shigley 2009 ; Eaton-Magana et al. 2019 ). Diamonds of group D-F are those diamonds having no colour. This category comprises of only less than 1% of all the gem-quality diamonds. Group G-J diamonds have minor traces of colour that can be identified by well-trained graders. Group K-M diamonds possess a faint yellow or brown tint. These diamonds represent about 40% of all the gem-quality diamonds (Hainschwang et al. 2006 ; Fridrichová et al. 2015 ). Natural diamonds are formed in the Earth’s crust under high pressure and high temperature (HPHT) conditions. In these conditions, most of the materials in the mantle are in the fluid state from which diamonds get crystallised under favourable conditions. These conditions might also facilitate entry of the similar size atoms/ions into diamond fluid and when crystallisation takes place, they could remain in the diamond crystal. Thus, during crystallisation under HPHT conditions, it is feasible to insert lattice defects like hetero atoms, vacancies and interstitials into the carbon lattice. Diamond (sp 3 ), Graphite (sp 2 ) and Fullerene (sp 2 + sp 3 ) are allotropes of carbon (Boyd et al. 1994 ; Pezzagna et al. 2011 ; Lee et al. 2020 ). Pure diamond is a carbon only crystal with sp 3 hybridisation. Figures 1 a & 1 b illustrate the tetrahedral arrangement of carbon atoms in the diamond crystal with each carbon atom having four valence electrons. The sp 3 hybridized carbon (C) in a diamond crystal forms four covalent bonds with neighbouring carbon atoms and C-C bond length is 154 pm (Rabinowitz et al. 2021a ). Nitrogen and Boron, being neighbours of carbon in the Periodic Table, could enter in the diamond lattice during its formation, if present in its vicinity, and results in bond formation with C atoms of the diamond lattice. The C-N bond length is 145 pm (Rabinowitz et al. 2021a ) compared to C-C bond length of 154 pm. Generally, N replaces C in the lattice of the diamond and because of variation in bond lengths, it creates structural asymmetry. Presence of nitrogen imparts yellow colour to diamonds whereas boron imparts blue colour. Intensity of colour of the diamond depends on the concentration of either of these atoms. A large number of color centers exist in type Ia diamonds that show absorption in UV-Vis and IR regions. Some of them also can be probed using Raman spectroscopy. Nitrogen impurity could possibly be present as single substituted N known as C-centre or three N atoms surrounding a vacancy known as N3 centre (Fridrichová et al. 2015 ; Rabinowitz et al. 2021a ). Yellow colour of type Ia diamonds is attributed mainly to the presence of N3 centre and shows an absorption peak at 415 nm (2.98 eV). This is due to electronic transition. Type Ia diamonds may also show an absorption peak at 478 nm (2.59 eV) known as N2 center which becomes observable in the absorption spectrum with increase in nitrogen concentration in the diamonds. Although 415 nm absorption is due to electronic transition, but interaction between electronic structure and vibronics continuum of diamond might give broad vibronic side band. In the case of IR region, for type Ia diamonds, nitrogen related absorptions are dominated mostly involving A, B and C centers. A center having two nitrogens together (N2) has an absorption at 1282 cm − 1 (0.159 eV). Intensity of this peak is directly proportional to the concentration of nitrogen of A center. Whereas B center, which is an aggregate of 4 nitrogens around a vacancy (N 4 V) shows peaks at 1172 cm − 1 (0.146 eV). Presence of isolated N, is known as C center (N s 0 ). It shows absorption peaks at 1344 (0.167 eV) and 1130 cm − 1 (0.140 eV) (Green et al., 2022 ). Apart from the impurities, the diamond crystal might have interstitial defects consequent to dislocation of carbon atoms. These dislocated carbon atoms aggregate and are commonly known as platelets (Olivier et al. 2018 ). It is reported that increase in nitrogen concentration plays an important role in the orientation of carbon-based platelet defects by inducing mechanical stress (Woods G. S. 1986; Olivier et al. 2018 ). Since more N atoms in a diamond might increase stress on platelets that would affect vibrational energy frequency as well as width of the platelet peak (FWHM). On this premise, we explored in the present work to correlate both platelet peak position and its FWHM to colour grade of the type-Ia diamonds. Colour grading of a diamond is routinely determined in Gemmological institute of India (GII) by more than one experienced graders following the established set protocol. Diamond is examined under a D65 light source with the naked eye and compared it with a set of reference, either standards of GIA or CIBJO. Both the test and reference diamonds are to be examined under identical and standard conditions of light (D’Haenens-Johansson et al. 2014 ; ISO 24016 2020 ). The methodology is standardised by GIA and universally followed. This method requires training, skill and practice, and the final results are normally reliable but subjective because it relies on the grader’s judgement instead of a numerical set of values or a graphical method. It is, therefore, desirable to have an instrumental method for determining the colour grades of the diamonds with defined precision and reliability. Methods based on using optical spectroscopic techniques are promising for this (D’Haenens-Johansson et al. 2014 ). Presence of impurities and crystal defects in a diamond generally downgrades the value of that diamond. Investigations on their presence might provide clues for understanding crystal structure and possible characteristic energy transitions corresponding to emissions/absorptions in the IR and UV-Vis. regions which could be useful in spectroscopic characterisation. Measurements based on optical spectrometric methods are useful to get relevant information on the defects that could be associated with colour grade. Instrumental spectrometric methods like IR, UV-Vis. and Raman both in scattering and photoluminescence (PL) modes are useful to obtain absorption/emission spectra of the diamonds (Palyanov et al. 2010 ; ISO 24016 2020 ) in ascertaining the grades of the diamonds. A few reports are available in the literature on the instrumental methods developed for determining the colour grades of diamonds with varying degrees of success.Rabinowitz et al. ( 2021a ) used millimetre wave spectroscopic properties of diamonds to determine the colour grades of diamonds. Their results on free spectral range plots (FSR) were used to correlate with the amount of nitrogen present in the diamond investigated which in turn was used to determine the colour grade. This method yielded 30 % false-true results indcating that colour grades of 70 % of diamonds measured wre reproduced and reliable. These authors experimentally arrived at an optimum frequency range of 100–110 GHz (W band) from their studies for correlating with colour grades using 26 diamonds as test samples.Lee et al., ( 2020 ) reported EPR (Electron Paramagnetic Resonance) measurements of unpaired nitrogen in GIA standard diamonds (Rabinowitz et al. 2021a ). They arrived at a correlation between the measured absorption intensity peak at 415 nm corresponding to N3 center and concentration of nitrogen present in these diamonds obtained by measuring unpaired electrons of nitrogen by EPR spectroscopy (Lee et al. 2020 ). Thus, they could correlate measured UV-Vis. absorption intensities with the colour grade of the same GIA standards set and obtained correlation that could be used for grading of diamonds.Rabinowitz et al. ( 2021b ) measured electronic susceptibility for GIA standard diamonds by electromagnetic measurements in the microwave frequency range of 3.95 to 26.5 GHz, as electronic susceptibility is due to unpaired electrons in nitrogen-based impurities. The authors obtained a correlation between measured electronic susceptibility and colour grades of the diamonds (Rabinowitz et al. 2021b ). Some reports are available in the literature(García-Torres J.M. 2018; Miae et al. 2022 ) using advanced data science-based methods such as neural networks and machine learnings.Eaton-Magana & Breeding ( 2016 ) measured photoluminescence (PL) emission for a large number of IIa natural diamonds using 514 nm laser as an excitation source. The plots of PL peaks at 535.8 nm, 575 nm (NV 0 ) and 637 nm (NV − ) of natural Type IIa diamonds against normalised second order diamond Raman line (596 nm) intensity were used. Plots showed the variation of PL peak counts with respect to the different colour grades from D to L. It appears from the literature survey that colour grade determination of diamonds by instrumental methods is of current interest. It could also be seen from the literature that several researchers have attempted to establish a quantifiable and reproducible relation between nitrogen concentration and colour grades. Both model based theoretical calculations and experimental measurements were used to arrive at total nitrogen concentration which is responsible for the colour of the diamond. The success rate obtained by various researchers is not consistent and the relations reported in most of the cases are not straight forward. Thus, the field of developing an instrumental method for colour grading continues to be scientifically challenging and has a vast scope for exploration. Optical techniques like UV-Vis. spectrometry mainly for measuring the signal corresponding to N3 centre and IR measurements for apportioning and quantifying various nitrogen centres, are of great value. In this work, we have performed IR and UV-Vis. measurements on a set of pre-graded and validated diamonds that are regularly used in GII laboratory as standards (GIA standards master set). Platelet peak in IR spectrum provided a few co-relatable relations with colour grades from E to K. Peak position (frequency) shifted to higher frequency with colour grade (increasing nitrogen content) and its width (FWHM) got broadened with some exception for L and M grades. For L grade diamonds, platelet region became complex which was deconvoluted; details are given in results and discussion section. Results obtained on peak position, its width and intensity are evaluated for possible correlations with the grades of the diamonds. A total 336 of polished natural diamonds of type Ia, in the weight range from 0.3 to 5.0 carats were tested by this proposed methodology and the success rate for 135 out of 336 diamonds is good whereas for others deviations are large and thus unacceptable. Considering the cost and ease of measurements, we thought that it would be good to develop a simple methodology for colour grading of natural diamonds through UV-Vis. measurements. UV-Vis. absorption measurements were carried on the standards to arrive at correlation and another set of samples to evaluate the grade by this method. In the UV-Vis absorption spectra of these diamonds, it was observed that the intensity of N3 peak increased with the colour grade from E to M. Trends are very encouraging. Materials and Methods Nine RBC (Round Brilliant Cut) cut GIA diamond standards, were used as samples in the present measurements. Colour grades, Clarity and Carat weight of these diamonds are given in Table 1 . Table 2 shows diamonds (standards and test samples) for which spectral measurements were carried out. Table 1 Colour, clarity and carat weight of GIA standard set of diamonds used in this work are as follows. Sr. No. Weight in Carats Colour Grade Clarity 1 0.726 E VS1 $ 2 0.780 F VS2 $ 3 0.706 G VVS2 # 4 0.718 H VVS2 # 5 0.701 I VVS2 # 6 0.709 J VS1 $ 7 0.702 K VS1 $ 8 0.721 L VS1 $ 9 0.741 M VS1 $ #VVS1/VVS2: Very, very slightly included; $ VS1/VS2: Very slightly included. Table 2 Details of the diamonds studied in this work. Sample Weight Range (carats) No. of Diamonds Cut FTIR UV-Vis GIA standards 0.70–0.78 9 RBC √ √ Test sample set − 1 0.54–1.00 118 RBC √ X Test sample set − 2 0.50–1.00 207 RBC √ X Test sample set − 3 0.30–0.50 5 RBC √ √ Test sample set − 4 0.35–5.04 6 Fancy √ √ Test sample set − 5 0.70–0.93 8 RBC √ √ IR absorption spectra of diamonds listed in Table 2 were measured using Diffuse Reflectance Accessory (DRIFT) of Thermo Scientific’s Nicolet iS-50 with Deuterated L-alanine doped Triglycine Sulfate (DLaTGS) detector at room temperature. This set up has a resolution of 4 cm − 1 and spectra were recorded in the wavenumber (cm − 1 ) range from 400 cm − 1 to 6000 cm − 1 . The inbuilt programme has a provision to acquire background spectrum first, which will be subtracted from sample spectrum. UV-Vis NIR absorption spectra of the diamonds were acquired using Cary 5000, Agilent Technologies in the wavelength range from 200 nm to 1000 nm. The Cary Varian 5000 UV-Vis-NIR spectrometer having sources of Deuterium lamp for UV region and Tungsten Halogen for Visible and IR regions. This system has a PMT (photo multiplier tube) as the detector for UV-Visible region (190–800 nm) and polycrystalline lead sulfide (PbS) detector for IR region (800–1000 nm). Diamond was placed in the sample slot such that diamonds girdle touch the base of the sample slot of the instrument. Results and Discussions FTIR Measurements IR spectra for all the diamonds of GIA colour standards were recorded (E to M, ca. Table 1 ). IR spectrum of grade E is given in Fig. 2 and all the prominent absorption peaks are labelled including nitrogen aggregation peaks at 1282 cm − 1 (A-center) and 1172 cm − 1 (B-center) along with platelet peak at 1361.42 cm − 1 . Nitrogen aggregation in a diamond occurs as it stays in Earth’s crust for a long period of time and nitrogen present in the diamond provides signature absorption peaks in IR and visible regions. It is well known that nitrogen atoms enter in the diamond crystal lattice at substitutional sites and a single nitrogen substitution in the diamond is known as C-centre (1130 cm − 1 ). Over millions of years, N atoms might aggregate and get transformed into clusters. When two nitrogen atoms form a cluster, it is called A-center (1280 cm − 1 ) and four nitrogen aggregate around a vacancy to form B-center (1172 cm − 1 ) (Evans T. and Qi Z. 1982 ). Formation of B-centers is associated with creating a carbon vacancy and simultaneous occupation of an interstitial space by the dislocated carbon atom (Olivier et al. 2018 ). Such interstitial carbon atoms aggregate to form a platelet (Woods G. S. 1986; Olivier et al. 2018 ). Figure 3 (a) shows IR spectra of all the GIA standard reference diamonds of colour grades and saturation seen in the region from 1332 to 1150 cm − 1 for F to M colour grades. For clarity each spectrum is offset by 1 unit on Y axis. It is well known that the intensity/absorbance of nitrogen aggregate peaks at 1282 and 1172 cm − 1 depends upon concentration of nitrogen (Woods et al. 1990 ). Absorbance in this region is strong because of high nitrogen concentration and characteristic peaks in this region were engulfed due to signal saturation. By inspecting the Fig. 3 (a) it appears that peak area corresponding to 484 cm − 1 is increasing with grade. However preliminary analysis w.r.t. to grades of the diamonds didn’t yield a good correlation and therefore this peak was not considered for further analysis. Platelet peaks are observed in the wavenumber region from 1360 cm − 1 to 1380 cm − 1 . With increasing nitrogen concentration in diamonds, it is observed that the platelet peak position shifted towards higher wavenumber. To visualise the platelet peak shift, relevant part of IR spectra in the region of 1350 to 1450 cm − 1 , for diamonds of colour grade from E to M are shown in Fig. 3 (b). It is seen from the Fig. 3 (b) that the platelet peak position of all the measured diamonds are in the range from 1361 to 1370 cm − 1 . The platelet peak gets broadened for the diamonds from colour grade E to M and that is reflecting in the respective peak width (FWHM). From the measured IR spectra, values of the platelet peak position and FWHM were evaluated, and are given in Table 3 . Table 3 Peak position frequency and FWHM of platelet peak from measured IR spectra along with EPR measured unpaired nitrogen concentration {taken from (Lee et al. 2020 )} for all the diamond standards. Sr. No. Weight (Carats) Colour Grade Type Peak position (cm -1 ) FWHM (cm -1 ) N3 + C- centre signal (Lee et al. 2020 ) (ppm) 1 0.726 E IaA 1361.42 8.24 0.0008047 2 0.780 F Ia (saturated) 1364.45 10.24 0.0013584 3 0.706 G Ia (saturated) 1365.98 10.99 0.0036994 4 0.718 H Ia (saturated) 1365.26 10.95 0.0061923 5 0.701 I Ia (saturated) 1368.86 12.43 0.0111028 6 0.709 J Ia (saturated) 1370.50 13.35 0.0180881 7 0.702 K Ia (saturated) 1370.19 13.25 0.0355232 8 0.721 L Ia (saturated) 1366.20, 1362.79(split) 14.68 (combined value) 0.0581547 9 0.741 M Ia (saturated) 1367.33 13.95 0.0711033 FWHM has increased from grade E to M as shown in Table 3 . In the spectrum of grade L diamond, a doublet was observed (Fig. 3 (b)) in the platelet region. Deconvolution of the platelet peak was done by using ORIGIN 9.5 programme for L grade. A sharp peak at 1366.20 cm − 1 with an FWHM of 1.63 cm − 1 and a broad peak of 1362.79 cm − 1 with an FWHM of 13.05 cm − 1 were obtained. Sum of both the FWHMs is taken for the analysis and also given in the Table 3 . However, it is not clear whether the platelet peaks for other diamonds are multiplets or not. Deconvolution of the platelet peaks for all IR spectra of the diamonds did not show any trend. Nitrogen concentration values were taken from Lee et al., ( 2020 ) who estimated N concentration from their measurement of unpaired nitrogens by EPR and values are also given in Table 3 . Although measured values are for unpaired electron containing nitrogen atoms, they reported a correspondence to total nitrogen present in the diamonds. This variation of N concentration could affect planar orientation within the platelet. Increasing nitrogen content in the diamonds could increase the peak position frequency of the platelet which might be the consequence of the stress it causes on the platelet. This stress could result in the fluctuation in vibrational energy affecting the shape and position of the observed peak. For larger concentration of N centres, electron density increases with increasing N atoms (Woods G. S. 1986). To understand this, we also examined variation of FWHM of the platelet peak as a function of nitrogen concentration in the diamonds. Olivier et al., ( 2018 ) carried out monolayer imaging studies on type Ia diamonds and found that the concentration of nitrogen varied from 6 to 61% in a monolayer in the plane [100]. As the main objective of this work is to arrive at correlations between colour grade of standards and observable parameter like platelet peak position or platelet peak width from IR spectra or absorption intensities from UV-Vis measurements, measured data were analysed as described in the following. A scatter plot of platelet peak position versus the colour grade, giving arbitrary numerical values from 2 to 10 for the grades from E to M respectively, is shown in Fig. 4 . Similar arbitrary units for diamond colour grades were used by Lee et al., ( 2020 ) for correlating EPR data with the colour grades of the diamond. Although the peak position (frequency) increases with respect to the grade, the shift is less pronounced for the grades H and L, and deviates from the trend. Woods G. S. (1986) had observed pronounced scattering in the plot of platelet peak position as a function of absorption coefficient for a large number of diamonds which is a deviation from a straight line relation given by Davies G., ( 1977 ). Presence of nitrogen inside the platelets through the formation of a C-N bond could also affect the vibrational energy (Woods G. S. 1986). It is well known that the vibration frequency is inversely proportional to the reduced mass of vibrating molecule and is directly proportional to the bond strength. Presence of C-N bond (bond strength = 340 kJ/mol.) therefore might shift the characteristic band corresponding to platelet to higher value. Thus, it is possible that vibrational absorption peaks could shift to higher wavenumbers. This argument suggests that an increase in overall N concentration in the crystal might lead to a disorientation of the platelet structure in a way that creates stronger C-N bond within the platelet (Woods G. S. 1986; Sumiya et al. 2023 ). However, if this logic is applicable, then a monotonous increase could have been observed contrary to the present result and as well reported by Woods G. S., (1986). To examine for a possible correlation of platelet peak position frequency with the diamond colour grade, data were fitted to a polynomial giving arbitrary numerical values from 2 to 10 for the grades from E to M respectively and excluding outliers for grade L and M (ca. in Table 3 ). The fitted curves are also shown in Fig. 4 . Our data are well represented by a second order equation (black colour line) as well as by a linear equation (red colour line). R 2 values obtained for both the fits are respectively, 0.91 ± 0.01 for linear fit and 0.92 ± 0.02 for second order fit. The R 2 values indicate that both the fits are “fit for the purpose” and could be used for evaluating the colour grade of the polished diamonds. It is possible that the platelets could be undergoing a constant stress-strain phenomenon with respect to the host diamond crystal. Hence as the grade varies from E to M, increasing concentration of nitrogen might enhance the crystal stress encountered by the interplanar platelet defects. As a consequence, fluctuations of vibrational energy levels might be occurring (Sumiya et al. 2023 ) resulting in change in the peak width, as reported by Woods G. S., (1986). Our data on FWHM were analysed to examine for a possible relation between FWHM of the platelet peak and diamond colour grade. Figure 5 is a plot of FWHM of the platelet peak as a function of colour grade of the diamonds using arbitrary units (A.U.) like earlier. This Figure also contains best fitted lines. The FWHM values are increasing from E to M grades. From Fig. 5 , it is clear that both the fits are good with R 2 values of 0.96 ± 0.02 and 0.92 ± 0.02 respectively for second order fit (black colour) and linear fit (red colour line). The fitted curves are used to evaluate grades for diamonds of different sizes. As given in Table 2 , IR spectra for 336 diamond test samples were measured to determine their grade using the correlations obtained in the present methodology. From the IR spectra, peak position and FWHM of the platelet peak were evaluated. Grades of the test samples were evaluated using platelet peak position in the equation obtained by using the linear fit (Fig. 5 ). These values are compared with the grades determined using conventional method by GII graders. Giving numerical values to grades, the differences between GII grades and evaluated grades by the present spectrometric method are given in Table 4 . For 135 diamonds, variation is within one grade. For 101 diamonds variation is within 2 grades and for another 80 diamonds variation is within 3 grades whereas deviation is higher for(Lee et al. 2020 ) 14 diamonds. Grades of these 336 diamonds have also been evaluated using second order fit parameters and results obtained are given in column 4. Deviations for about 200 diamonds from both the sets of results (given in columns 3 & 4) are 2 or more grades and therefore this methodology needs to be evaluated further to consider for developing a colour grading methodology. Also to obtain better statistics a large number of test samples are to be measured. Table 4 Deviations of grades of test diamond samples determined by the present method compared to the grades determined by GII graders. Sr. No. Colour grades deviation No. Samples Using 1st order Using 2nd order 1 0 to ± 1 135 141 2 ± 1 to ± 2 101 112 3 ± 2 to ± 3 80 63 4 ± 3 < 14 14 Since presence of nitrogen gives colour tinge to diamonds, absorbance in the visible region (415 nm) was measured. An attempt is made to examine the possible correlation between platelet peak position and total N concentration taken from Lee et al., ( 2020 ). Figure 6 shows a plot between platelet peak position obtained from IR spectra of GIA standards and nitrogen concentration from EPR measurements by Lee et al., ( 2020 ). Although data fit well to a second order equation, it does not explain why two different nitrogen concentrations have same platelet frequency. Thus, it gives two different grades and vice versa. This suggests that platelet peak position is not a good parameter for diamond colour grade determination. Figure 7 shows a fitted curve between the FWHM of the platelet peak positions as a function of nitrogen concentration (N3 + C). The fit follows a polynomial given in Eq. (1), $$Y=-958 {X}^{2}+147.40 X+9.10 \left(1\right)$$ Where, Y is FWHM of the platelet and X is N concentration taken from Lee et al., ( 2020 ). Both the fits show nearly similar range of R 2 values of 0.91 ± 0.01 and 0.85 ± 0.02 respectively. Thus, it could be said that both FWHM and peak positions showed a reasonable correlation with the nitrogen concentration. UV-Vis NIR Measurements As described earlier, in the process of transformation of ‘A’ cluster into ‘B’ cluster, it is possible to create a three nitrogen atom cluster around a vacancy known as N3 centre (Woods G. S. 1986; Olivier et al. 2018 ). This center absorbs blue light in the visible region giving a peak at 415 nm. Yellow tint of the diamond increases with increasing grade and it reflects in an increase in the intensity of the peak at 415 nm We adopted the nitrogen concentrations for GIA standards from Lee et al., ( 2020 ). As described in the introduction section, we thought of developing a simple method of colour grading through UV-Vis. measurements of diamonds. For that reason, we also analysed our samples with a UV-Vis. absorption spectrometer. As colour is the major criterion for deciding colour grade, UV-Vis. absorption spectral measurements were made. Absorption data were analysed to examine a plausible relation between absorption and colour grade. We recorded UV-Vis. absorption spectra for all the GIA colour standards at room temperature under similar experimental conditions. All the GIA standards have nearly same weight and RBC cut as given in Table 1 . According to Beer-Lambert law, absorbance is directly proportional to the path length through which source light beam passes through the material. For RBC cut diamond, the longest path is along the girdle, and therefore diamond is placed on girdle position. Figure 8 shows the UV-Vis absorption spectra of all GIA standards. Lee et al., ( 2020 ), besides measuring concentration of single nitrogen atoms with unpaired electron by EPR spectroscopy, also obtained absorbance corresponding to N3 (415 nm) and C (477 nm) centre by UV-Vis spectroscopy.Lee et al., ( 2020 ) established a good correlation between unpaired nitrogen concentration and colour of the diamond. It is interesting to note that nitrogen concentration correlated well with the trends seen with UV-Vis (electronic transition) absorption data, common link being the colour grade. We explored to examine whether N3 Peak area correlates with the concentration of N3 + C centre given by Lee et al., ( 2020 ) (Fig. 9 ). The derived analytical function appears to suggest peak area of N3 centre might be a good parameter to determine a colour grade. Figure 10 shows a scatter plot of N3 (415 nm) peak area of GIA standards and test sample set – 5 with respective colour grades. The plot for Sample set-5 (red filled circles) shows nearly similar trend as that of GIA standards (Black squares) from higher to lower colour grades. The peak areas of N3 centre also show similar trend with continuous increase in the peak area with increasing colour grade except for grade K. This is an encouraging sign to develop a methodology that could be converted to fabricate a machine to determine the colour grades of polished diamonds by measuring the absorbance in N3 window using visible/colour spectrometer. Total absorbance in the visible region appears to be another reliable parameter to correlate colour grading of a diamond. However, we have to test more samples for further examining this methodology. Conclusions Platelet peak position and its FWHM from measured IR spectra of GIA standard diamonds were evaluated. Using the fit parameters obtained from plots between peak position and colour grade of the standards, colour grades for 336 diamonds of different sizes were evaluated. Although colour grades between thus computed values by present method and determined by GII graders are in reasonable agreement, deviation for 195 diamonds are two or more grades. Therefore, this methodology cannot be accepted as reliable. On the other hand, on other hand a good correlation between platelet position measured by us and N3 + C from literature was obtained for the same set of diamonds but the double values for the same platelet frequency does not encourage to adopt this methodology. Further, visible absorbance of GII standards gave a good correlation with the grades. Measured absorbance of a set of diamonds of similar size, compared well with the trends obtained with GIA standards. We are hopeful that a methodology could be developed for colour grading of diamonds using measurement of absorbance in the visible region that would lead to fabricate a colour grading machine by incorporating a simple visible absorption spectrometer. Declarations Author Contributions Conceptualization, R.A.; B.J. and M.G.; methodology, R.A.; B.J. and M.G.; software, R.A.; validation, R.A.; B.J.; M.G. and A.V.R.; formal analysis, R.A.; investigation, R.A.; and A.V.R.; resources, R.A.; A.G.; and A.V.R.; data curation, R.A.; and A.G.; writing—original draft preparation, R.A.; A.G.; M.G.; S.M.; and A.V.R.; writing—review and editing, R.A.; A.G.; M.G.; S.M.; H.B.; and A.V.R. All authors have read and agreed to the published version of the manuscript. Availability of data and materials No. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements Authors thank Dr. M.D. Sastry former Head, R&D, GII for initiating this work and Mr. Yogesh Salunkhe for help in characterizing the diamonds using confocal Raman microscopy details of which are not added in this manuscript. References Bouman M, Anthonis A, Chapman J, et al (2018) The Effect of Blue Fluorescence on the Colour Appearance of Round-Brilliant-Cut Diamonds. The Journal of Gemmology 36:298–315. https://doi.org/10.15506/JoG.2018.36.4.298 Boyd SR, Kiflawi I, Woods GS (1994) The relationship between infrared absorption and the A defect concentration in diamond. Philosophical Magazine B 69:1149–1153. https://doi.org/10.1080/01418639408240185 Breeding CM, Eaton‐Magaña S (2019) Fluorescence of Natural and Synthetic Gem Diamond: Mechanism and Applications. In: Encyclopedia of Analytical Chemistry. Wiley, pp 1–26 Breeding CM, Eaton-Magana S, Shigley JE (2020) Naturally Colored Yellow and Orange Gem Diamonds: The Nitrogen Factor. Gems & Gemology 56:194–219. https://doi.org/10.5741/GEMS.56.2.194 Breeding CM, Shigley JE (2009) The “Type” Classification System of Diamonds and Its Importance in Gemology. Gems & Gemology 45:96–111. https://doi.org/10.5741/GEMS.45.2.96 Davies G. (1977) Chemistry and Physics of Carbon. Dekker, New York, USA D’Haenens-Johansson UFS, Soe Moe K, Johnson P, et al (2014) Near-Colorless HPHT Synthetic Diamonds from AOTC Group. Gems & Gemology 50:30–45. https://doi.org/10.5741/GEMS.50.1.30 Eaton-Magana S, Ardon T, Breeding CM, Shigley JE (2020) Natural-Color D-to-Z Diamonds: A Crystal-Clear Perspective. Gems & Gemology 56:318–335. https://doi.org/10.5741/GEMS.56.3.335 Eaton-Magana S, Ardon T, Breeding CM, Shigley JE (2019) Natural-Color Fancy White and Fancy Black Diamonds: Where Color and Clarity Converge. Gems & Gemology 55:. https://doi.org/10.5741/GEMS.55.3.320 Eaton-Magaña S, Ardon T, Smith K V., et al (2019) Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors. Gems & Gemology. https://doi.org/10.5741/GEMS.54.4.352 Eaton-Magana S, Breeding CM (2016) An Introduction to Photoluminescence Spectroscopy for Diamond and its Applications in Gemology. Gems & Gemology 52:2–17. https://doi.org/10.5741/GEMS.52.1.2 Eaton-Magaña S, Breeding CM, Shigley JE (2018) Natural-color blue, gray, and violet diamonds: Allure of the deep. Gems and Gemology 54:112–131. https://doi.org/10.5741/GEMS.54.2.112_i Evans T., Qi Z. (1982) The kinetics of the aggregation of nitrogen atoms in diamond. Proceedings of the Royal Society of London A Mathematical and Physical Sciences 381:159–178. https://doi.org/10.1098/rspa.1982.0063 Fridrichová J, Bacik P, Škoda R, Antal P (2015) Use of spectroscopic methods for determination of diamond origin and treatment. Acta Geologica Slovaca 7:11–18 García-Torres J.M. (2018) Implementing a deep learning algorithm for diamond classification Green BL, Collins AT, Breeding CM (2022) Diamond Spectroscopy, Defect Centers, Color, and Treatments. Rev Mineral Geochem 88:637–688. https://doi.org/10.2138/rmg.2022.88.12 Hainschwang T, Notari F, Fritsch E, Massi L (2006) Natural, untreated diamonds showing the A, B and C infrared absorptions (“ABC diamonds”), and the H2 absorption. Diam Relat Mater 15:1555–1564. https://doi.org/10.1016/j.diamond.2005.12.029 ISO 24016 (2020) Jewellery and precious metals grading polished diamonds terminology, classification and test methods. In: ISO 24016. https://standards.iteh.ai/catalog/standards/iso/d625a4fd-e4d4-48f2-b136-4bd4d88303de/iso-24016-2020. Accessed 18 Sep 2020 King J.M., Shigley J.E., Gelb T.H., et al (2005) Unusually large novelty cut. Gems & Gemmology 88–111 Lee CWY, Cheng J, Yiu YC, et al (2020) Correlation between EPR spectra and coloration of natural diamonds. Diam Relat Mater 103:107728. https://doi.org/10.1016/j.diamond.2020.107728 Miae MA, Malykhina GF, Manev D (2022) Deep Learning Applications in Industrial Grading System. pp 431–441 Olivier EJ, Neethling JH, Kroon RE, et al (2018) Imaging the atomic structure and local chemistry of platelets in natural type Ia diamond. Nat Mater 17:243–248. https://doi.org/10.1038/s41563-018-0024-6 Palyanov YN, Borzdov YM, Khokhryakov AF, et al (2010) Effect of Nitrogen Impurity on Diamond Crystal Growth Processes. Cryst Growth Des 10:3169–3175. https://doi.org/10.1021/cg100322p Pezzagna S, Rogalla D, Wildanger D, et al (2011) Creation and nature of optical centres in diamond for single-photon emission—overview and critical remarks. New J Phys 13:035024. https://doi.org/10.1088/1367-2630/13/3/035024 Rabinowitz Y, Etinger A, Litvak B, et al (2021a) Millimeter wave spectroscopy for evaluating diamond color grades. Diam Relat Mater 116:108386. https://doi.org/10.1016/j.diamond.2021.108386 Rabinowitz Y, Etinger A, Yahalom A, et al (2021b) Microwave Spectroscopy as a Potential Tool for Color Grading Diamonds. Energies (Basel) 14:3507. https://doi.org/10.3390/en14123507 Sumiya H, Sakano F, Tatsumi N (2023) Distribution of internal strain and fracture strength in various single-crystal diamonds. Diam Relat Mater 134:109781. https://doi.org/10.1016/j.diamond.2023.109781 Woods G. S. (1986) Platelets and the infrared absorption of type Ia diamonds. Proceedings of the Royal Society of London A Mathematical and Physical Sciences 407:219–238. https://doi.org/10.1098/rspa.1986.0094 Woods GS, Purser GC, Mtimkulu ASS, Collins AT (1990) The nitrogen content of type Ia natural diamonds. Journal of Physics and Chemistry of Solids 51:1191–1197. https://doi.org/10.1016/0022-3697(90)90101-K Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4593034","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":323813592,"identity":"0645d0ed-609b-45e8-8984-1c8c4d9be67a","order_by":0,"name":"Rajendra Ardalkar","email":"","orcid":"","institution":"Gemmological Institute of India","correspondingAuthor":false,"prefix":"","firstName":"Rajendra","middleName":"","lastName":"Ardalkar","suffix":""},{"id":323813593,"identity":"1ba047c8-5ba1-4d08-9a34-2b8d7cf6044b","order_by":1,"name":"Sandesh Mane","email":"","orcid":"","institution":"Gemmological Institute of India","correspondingAuthor":false,"prefix":"","firstName":"Sandesh","middleName":"","lastName":"Mane","suffix":""},{"id":323813594,"identity":"7222b429-96ad-4b6f-9ba5-27686f7b5f95","order_by":2,"name":"Anik Goswami","email":"","orcid":"","institution":"Gemmological Institute of India","correspondingAuthor":false,"prefix":"","firstName":"Anik","middleName":"","lastName":"Goswami","suffix":""},{"id":323813598,"identity":"20ca0de5-2ebd-4973-812a-c575cb64521f","order_by":3,"name":"Mahesh Gaonkar","email":"","orcid":"","institution":"Gemmological Institute of India","correspondingAuthor":false,"prefix":"","firstName":"Mahesh","middleName":"","lastName":"Gaonkar","suffix":""},{"id":323813600,"identity":"bb96d0ff-1872-4958-8548-6e2d8de01966","order_by":4,"name":"Bhavik Joshi","email":"","orcid":"","institution":"Gemmological Institute of India","correspondingAuthor":false,"prefix":"","firstName":"Bhavik","middleName":"","lastName":"Joshi","suffix":""},{"id":323813602,"identity":"54d36b40-2f1a-45ac-ac4f-67adc7fbedc3","order_by":5,"name":"Hemlata Bagla","email":"","orcid":"","institution":"K.C. College","correspondingAuthor":false,"prefix":"","firstName":"Hemlata","middleName":"","lastName":"Bagla","suffix":""},{"id":323813603,"identity":"9753762b-2dc6-4021-bd5b-4e89b9f9eaab","order_by":6,"name":"A. V.R. Reddy","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA/0lEQVRIiWNgGAWjYJADAwkGAxsgzdh4gAQtBWkgLQ2kaPlwGMzCq4V/Ru7DTzdq7sibtzdvvM1jcN5ubfthoC01NtG4tEjcSDeWzjn2zHDOmWPF1jwGt5O3nUkEajmWltuAS8+ZYwzSOWyHGWdI5JhJg7SYHQBqYWw4jFOL/JljzL9z/h22nyH/BqTlXLLZ+Yf4tRgcb2OTzm07nDhDggek5YCd2Q0CthgCtVjn9h1OnsGTVmw5xyA5wewG0JYEPH6RO8zGfDvn22HbGeyHN95488fO3ux8+sMHH2pscHsfGTDxMDAkglUmEKMcBBh/MDDYE6t4FIyCUTAKRg4AAP9dY/WP9XYIAAAAAElFTkSuQmCC","orcid":"","institution":"Gemmological Institute of India","correspondingAuthor":true,"prefix":"","firstName":"A.","middleName":"V.R.","lastName":"Reddy","suffix":""}],"badges":[],"createdAt":"2024-06-17 09:09:46","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4593034/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4593034/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":60091203,"identity":"71462cf6-ae52-434a-bbb6-8de4482f5445","added_by":"auto","created_at":"2024-07-11 16:11:11","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":34422,"visible":true,"origin":"","legend":"\u003cp\u003eArrangement of carbon atoms in the diamond crystal: (a) Crystal structure of diamond unit cell; (b) Tetrahedral arrangement of carbon atoms in the diamond lattice.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/8cd79abaa6ed5ba41f6862e9.jpg"},{"id":60091194,"identity":"6243b389-720f-4347-ae01-c6ffdd242587","added_by":"auto","created_at":"2024-07-11 16:11:09","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":79332,"visible":true,"origin":"","legend":"\u003cp\u003eIR absorption spectrum of the GIA standard diamond of colour grade E.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/90d6d24c0aa08782c0faaf69.jpg"},{"id":60091195,"identity":"7130b4fb-c456-48d1-93cc-38d6c6c87a8a","added_by":"auto","created_at":"2024-07-11 16:11:09","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":93648,"visible":true,"origin":"","legend":"\u003cp\u003e(a) IR spectra of all the GIA diamond standards; (b) platelet peak region (1350 to 1450 cm\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/9ca8b191e84c05b21f3bb4a6.jpg"},{"id":60091200,"identity":"9d8a1ebc-f280-4c49-9d0f-bd19cad1dd38","added_by":"auto","created_at":"2024-07-11 16:11:10","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":90506,"visible":true,"origin":"","legend":"\u003cp\u003eScatter plot of platelet peak position vs color grades of GIA standard diamonds along with linear and second order fitted curves.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/dbdfbd2c62f55e6940c583f1.jpg"},{"id":60091201,"identity":"ddbbc67c-dea3-4ca1-bcc4-5a9a19f151e8","added_by":"auto","created_at":"2024-07-11 16:11:10","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":51365,"visible":true,"origin":"","legend":"\u003cp\u003ePlot of FWHM of the platelet peaks vs color grades of GIA standard diamonds.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/c336d09129bcc8ab38cf6e5b.jpg"},{"id":60091202,"identity":"dc80e8b5-73ae-4c7c-8f0f-4ae9801e1d94","added_by":"auto","created_at":"2024-07-11 16:11:11","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":43883,"visible":true,"origin":"","legend":"\u003cp\u003eA plot of platelet peak position vs (N3 +C) signal from EPR (Lee et al. 2020).\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/72067a1b8d155965a2003fdd.jpg"},{"id":60091207,"identity":"54d39c7b-3907-43c6-87fa-a393f89b23d6","added_by":"auto","created_at":"2024-07-11 16:11:13","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":38019,"visible":true,"origin":"","legend":"\u003cp\u003eA plot of FWHM of platelet peak vs (N3 +C) signal from EPR (Lee et al. 2020).\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/a4e42facef2bd799d045ac74.jpg"},{"id":60091204,"identity":"96a0e2df-c11f-4f29-b446-bbafe1723e2b","added_by":"auto","created_at":"2024-07-11 16:11:11","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":75198,"visible":true,"origin":"","legend":"\u003cp\u003eUV-Vis absorption spectra of all GIA diamond standards. Inset shows N3 absorption peak at 415 nm and C- centre absorption peak at 477 nm.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/bfc389a9c09bbe65aa18f551.jpg"},{"id":60091205,"identity":"f8037543-f725-462a-a149-1b929fb05249","added_by":"auto","created_at":"2024-07-11 16:11:12","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":28386,"visible":true,"origin":"","legend":"\u003cp\u003eA plot of peak area of N3 (415 nm) in UV-Vis absorption vs (N3 +C) signal from EPR (Lee et al. 2020).\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/8d4ad7829a94be1f66697153.jpg"},{"id":60091206,"identity":"03f52fbb-dbdf-4f84-a1a8-efc358dd59e0","added_by":"auto","created_at":"2024-07-11 16:11:13","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":32969,"visible":true,"origin":"","legend":"\u003cp\u003eA plot of peak area of N3 centre (415 nm) of GIA std. and test sample set-5 in UV-Vis absorbance vs colour grade.\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/ff929ffe7ee00080502b8d50.jpg"},{"id":61527888,"identity":"11a98aba-f06f-4645-b11e-377b0f62e66a","added_by":"auto","created_at":"2024-07-31 21:08:48","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1128609,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4593034/v1/04c92ea3-cb6a-4dc8-9f63-d7b9c43ca534.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eSpectroscopic Evaluation of Colour Grading of Type Ia Diamonds†.\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eNatural diamonds are evaluated to enhance the scientific understanding of their diverse colour variations, unusual inclusions and cutting flaws associated with them. Natural diamonds are highly sought after for their brilliance and commercial value. Their value is determined by well-known 4Cs namely Colour, Clarity, Cut and Carat weight (Bouman et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Breeding \u0026amp; Eaton-Maga\u0026ntilde;a, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Eaton-Magana et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). White colour diamonds are the best grade which is denoted by grade D. Other diamonds are graded lower than D (Eaton-Maga\u0026ntilde;a et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Breeding et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the past, various grading systems were used to denote the colour grading of diamonds. At present, GIA (Gemmological Institute of America) grading system as well as CIBJO (Conf\u0026eacute;d\u0026eacute;ration Internationale de la Bijouterie, Joaillerie, Orf\u0026egrave;vrerie) grading system are being followed internationally (Eaton-Maga\u0026ntilde;a et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). In both the systems, D (white) represents the best grade diamonds and Z represents the lowest grade diamonds (Eaton-Magana et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Colour grade scale for smaller diamonds (\u0026le;\u0026thinsp;0.15 carats) is divided into broad groups by GIA (King J.M. et al. 2005; Breeding and Shigley \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Eaton-Magana et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Diamonds of group D-F are those diamonds having no colour. This category comprises of only less than 1% of all the gem-quality diamonds. Group G-J diamonds have minor traces of colour that can be identified by well-trained graders. Group K-M diamonds possess a faint yellow or brown tint. These diamonds represent about 40% of all the gem-quality diamonds (Hainschwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Fridrichov\u0026aacute; et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eNatural diamonds are formed in the Earth\u0026rsquo;s crust under high pressure and high temperature (HPHT) conditions. In these conditions, most of the materials in the mantle are in the fluid state from which diamonds get crystallised under favourable conditions. These conditions might also facilitate entry of the similar size atoms/ions into diamond fluid and when crystallisation takes place, they could remain in the diamond crystal. Thus, during crystallisation under HPHT conditions, it is feasible to insert lattice defects like hetero atoms, vacancies and interstitials into the carbon lattice.\u003c/p\u003e \u003cp\u003eDiamond (sp\u003csup\u003e3\u003c/sup\u003e), Graphite (sp\u003csup\u003e2\u003c/sup\u003e) and Fullerene (sp\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;+\u0026thinsp;sp\u003csup\u003e3\u003c/sup\u003e) are allotropes of carbon (Boyd et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Pezzagna et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Lee et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Pure diamond is a carbon only crystal with sp\u003csup\u003e3\u003c/sup\u003e hybridisation. Figures\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea \u0026amp; \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb illustrate the tetrahedral arrangement of carbon atoms in the diamond crystal with each carbon atom having four valence electrons. The sp\u003csup\u003e3\u003c/sup\u003e hybridized carbon (C) in a diamond crystal forms four covalent bonds with neighbouring carbon atoms and C-C bond length is 154 pm (Rabinowitz et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNitrogen and Boron, being neighbours of carbon in the Periodic Table, could enter in the diamond lattice during its formation, if present in its vicinity, and results in bond formation with C atoms of the diamond lattice. The C-N bond length is 145 pm (Rabinowitz et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e) compared to C-C bond length of 154 pm. Generally, N replaces C in the lattice of the diamond and because of variation in bond lengths, it creates structural asymmetry. Presence of nitrogen imparts yellow colour to diamonds whereas boron imparts blue colour. Intensity of colour of the diamond depends on the concentration of either of these atoms.\u003c/p\u003e \u003cp\u003eA large number of color centers exist in type Ia diamonds that show absorption in UV-Vis and IR regions. Some of them also can be probed using Raman spectroscopy. Nitrogen impurity could possibly be present as single substituted N known as C-centre or three N atoms surrounding a vacancy known as N3 centre (Fridrichov\u0026aacute; et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Rabinowitz et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e). Yellow colour of type Ia diamonds is attributed mainly to the presence of N3 centre and shows an absorption peak at 415 nm (2.98 eV). This is due to electronic transition. Type Ia diamonds may also show an absorption peak at 478 nm (2.59 eV) known as N2 center which becomes observable in the absorption spectrum with increase in nitrogen concentration in the diamonds. Although 415 nm absorption is due to electronic transition, but interaction between electronic structure and vibronics continuum of diamond might give broad vibronic side band. In the case of IR region, for type Ia diamonds, nitrogen related absorptions are dominated mostly involving A, B and C centers. A center having two nitrogens together (N2) has an absorption at 1282 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (0.159 eV). Intensity of this peak is directly proportional to the concentration of nitrogen of A center. Whereas B center, which is an aggregate of 4 nitrogens around a vacancy (N\u003csub\u003e4\u003c/sub\u003eV) shows peaks at 1172 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (0.146 eV). Presence of isolated N, is known as C center (N\u003csub\u003es\u003c/sub\u003e\u003csup\u003e0\u003c/sup\u003e). It shows absorption peaks at 1344 (0.167 eV) and 1130 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (0.140 eV) (Green et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eApart from the impurities, the diamond crystal might have interstitial defects consequent to dislocation of carbon atoms. These dislocated carbon atoms aggregate and are commonly known as platelets (Olivier et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). It is reported that increase in nitrogen concentration plays an important role in the orientation of carbon-based platelet defects by inducing mechanical stress (Woods G. S. 1986; Olivier et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Since more N atoms in a diamond might increase stress on platelets that would affect vibrational energy frequency as well as width of the platelet peak (FWHM). On this premise, we explored in the present work to correlate both platelet peak position and its FWHM to colour grade of the type-Ia diamonds.\u003c/p\u003e \u003cp\u003eColour grading of a diamond is routinely determined in Gemmological institute of India (GII) by more than one experienced graders following the established set protocol. Diamond is examined under a D65 light source with the naked eye and compared it with a set of reference, either standards of GIA or CIBJO. Both the test and reference diamonds are to be examined under identical and standard conditions of light (D\u0026rsquo;Haenens-Johansson et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; ISO 24016 \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The methodology is standardised by GIA and universally followed. This method requires training, skill and practice, and the final results are normally reliable but subjective because it relies on the grader\u0026rsquo;s judgement instead of a numerical set of values or a graphical method. It is, therefore, desirable to have an instrumental method for determining the colour grades of the diamonds with defined precision and reliability. Methods based on using optical spectroscopic techniques are promising for this (D\u0026rsquo;Haenens-Johansson et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePresence of impurities and crystal defects in a diamond generally downgrades the value of that diamond. Investigations on their presence might provide clues for understanding crystal structure and possible characteristic energy transitions corresponding to emissions/absorptions in the IR and UV-Vis. regions which could be useful in spectroscopic characterisation. Measurements based on optical spectrometric methods are useful to get relevant information on the defects that could be associated with colour grade. Instrumental spectrometric methods like IR, UV-Vis. and Raman both in scattering and photoluminescence (PL) modes are useful to obtain absorption/emission spectra of the diamonds (Palyanov et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; ISO 24016 \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) in ascertaining the grades of the diamonds.\u003c/p\u003e \u003cp\u003eA few reports are available in the literature on the instrumental methods developed for determining the colour grades of diamonds with varying degrees of success.Rabinowitz et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e) used millimetre wave spectroscopic properties of diamonds to determine the colour grades of diamonds. Their results on free spectral range plots (FSR) were used to correlate with the amount of nitrogen present in the diamond investigated which in turn was used to determine the colour grade. This method yielded 30 % false-true results indcating that colour grades of 70 % of diamonds measured wre reproduced and reliable. These authors experimentally arrived at an optimum frequency range of 100\u0026ndash;110 GHz (W band) from their studies for correlating with colour grades using 26 diamonds as test samples.Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) reported EPR (Electron Paramagnetic Resonance) measurements of unpaired nitrogen in GIA standard diamonds (Rabinowitz et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e). They arrived at a correlation between the measured absorption intensity peak at 415 nm corresponding to N3 center and concentration of nitrogen present in these diamonds obtained by measuring unpaired electrons of nitrogen by EPR spectroscopy (Lee et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Thus, they could correlate measured UV-Vis. absorption intensities with the colour grade of the same GIA standards set and obtained correlation that could be used for grading of diamonds.Rabinowitz et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e) measured electronic susceptibility for GIA standard diamonds by electromagnetic measurements in the microwave frequency range of 3.95 to 26.5 GHz, as electronic susceptibility is due to unpaired electrons in nitrogen-based impurities. The authors obtained a correlation between measured electronic susceptibility and colour grades of the diamonds (Rabinowitz et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSome reports are available in the literature(Garc\u0026iacute;a-Torres J.M. 2018; Miae et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) using advanced data science-based methods such as neural networks and machine learnings.Eaton-Magana \u0026amp; Breeding (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) measured photoluminescence (PL) emission for a large number of IIa natural diamonds using 514 nm laser as an excitation source. The plots of PL peaks at 535.8 nm, 575 nm (NV\u003csup\u003e0\u003c/sup\u003e) and 637 nm (NV\u003csup\u003e\u0026minus;\u003c/sup\u003e) of natural Type IIa diamonds against normalised second order diamond Raman line (596 nm) intensity were used. Plots showed the variation of PL peak counts with respect to the different colour grades from D to L.\u003c/p\u003e \u003cp\u003eIt appears from the literature survey that colour grade determination of diamonds by instrumental methods is of current interest. It could also be seen from the literature that several researchers have attempted to establish a quantifiable and reproducible relation between nitrogen concentration and colour grades. Both model based theoretical calculations and experimental measurements were used to arrive at total nitrogen concentration which is responsible for the colour of the diamond. The success rate obtained by various researchers is not consistent and the relations reported in most of the cases are not straight forward. Thus, the field of developing an instrumental method for colour grading continues to be scientifically challenging and has a vast scope for exploration. Optical techniques like UV-Vis. spectrometry mainly for measuring the signal corresponding to N3 centre and IR measurements for apportioning and quantifying various nitrogen centres, are of great value.\u003c/p\u003e \u003cp\u003eIn this work, we have performed IR and UV-Vis. measurements on a set of pre-graded and validated diamonds that are regularly used in GII laboratory as standards (GIA standards master set). Platelet peak in IR spectrum provided a few co-relatable relations with colour grades from E to K. Peak position (frequency) shifted to higher frequency with colour grade (increasing nitrogen content) and its width (FWHM) got broadened with some exception for L and M grades. For L grade diamonds, platelet region became complex which was deconvoluted; details are given in results and discussion section. Results obtained on peak position, its width and intensity are evaluated for possible correlations with the grades of the diamonds. A total 336 of polished natural diamonds of type Ia, in the weight range from 0.3 to 5.0 carats were tested by this proposed methodology and the success rate for 135 out of 336 diamonds is good whereas for others deviations are large and thus unacceptable.\u003c/p\u003e \u003cp\u003eConsidering the cost and ease of measurements, we thought that it would be good to develop a simple methodology for colour grading of natural diamonds through UV-Vis. measurements. UV-Vis. absorption measurements were carried on the standards to arrive at correlation and another set of samples to evaluate the grade by this method. In the UV-Vis absorption spectra of these diamonds, it was observed that the intensity of N3 peak increased with the colour grade from E to M. Trends are very encouraging.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003eNine RBC (Round Brilliant Cut) cut GIA diamond standards, were used as samples in the present measurements. Colour grades, Clarity and Carat weight of these diamonds are given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows diamonds (standards and test samples) for which spectral measurements were carried out.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eColour, clarity and carat weight of GIA standard set of diamonds used in this work are as follows.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSr. No.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eWeight in Carats\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eColour Grade\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eClarity\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.726\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVS1\u003csup\u003e$\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.780\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVS2\u003csup\u003e$\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.706\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eG\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVVS2\u003csup\u003e#\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.718\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVVS2\u003csup\u003e#\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.701\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVVS2\u003csup\u003e#\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.709\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eJ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVS1\u003csup\u003e$\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.702\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVS1\u003csup\u003e$\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.721\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVS1\u003csup\u003e$\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.741\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eVS1\u003csup\u003e$\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003e#VVS1/VVS2: Very, very slightly included; $ VS1/VS2: Very slightly included.\u003c/em\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDetails of the diamonds studied in this work.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eWeight Range (carats)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo. of Diamonds\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCut\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFTIR\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eUV-Vis\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGIA standards\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.70\u0026ndash;0.78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest sample set \u0026minus;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.54\u0026ndash;1.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e118\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eX\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest sample set \u0026minus;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.50\u0026ndash;1.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e207\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eX\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest sample set \u0026minus;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.30\u0026ndash;0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest sample set \u0026minus;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.35\u0026ndash;5.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFancy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest sample set \u0026minus;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.70\u0026ndash;0.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026radic;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIR absorption spectra of diamonds listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e were measured using Diffuse Reflectance Accessory (DRIFT) of Thermo Scientific\u0026rsquo;s Nicolet iS-50 with Deuterated L-alanine doped Triglycine Sulfate (DLaTGS) detector at room temperature. This set up has a resolution of 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and spectra were recorded in the wavenumber (cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) range from 400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 6000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The inbuilt programme has a provision to acquire background spectrum first, which will be subtracted from sample spectrum.\u003c/p\u003e \u003cp\u003eUV-Vis NIR absorption spectra of the diamonds were acquired using Cary 5000, Agilent Technologies in the wavelength range from 200 nm to 1000 nm. The Cary Varian 5000 UV-Vis-NIR spectrometer having sources of Deuterium lamp for UV region and Tungsten Halogen for Visible and IR regions. This system has a PMT (photo multiplier tube) as the detector for UV-Visible region (190\u0026ndash;800 nm) and polycrystalline lead sulfide (PbS) detector for IR region (800\u0026ndash;1000 nm). Diamond was placed in the sample slot such that diamonds girdle touch the base of the sample slot of the instrument.\u003c/p\u003e"},{"header":"Results and Discussions","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eFTIR Measurements\u003c/h2\u003e \u003cp\u003eIR spectra for all the diamonds of GIA colour standards were recorded (E to M, ca. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). IR spectrum of grade E is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and all the prominent absorption peaks are labelled including nitrogen aggregation peaks at 1282 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (A-center) and 1172 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (B-center) along with platelet peak at 1361.42 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNitrogen aggregation in a diamond occurs as it stays in Earth\u0026rsquo;s crust for a long period of time and nitrogen present in the diamond provides signature absorption peaks in IR and visible regions. It is well known that nitrogen atoms enter in the diamond crystal lattice at substitutional sites and a single nitrogen substitution in the diamond is known as C-centre (1130 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Over millions of years, N atoms might aggregate and get transformed into clusters. When two nitrogen atoms form a cluster, it is called A-center (1280 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and four nitrogen aggregate around a vacancy to form B-center (1172 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (Evans T. and Qi Z. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1982\u003c/span\u003e). Formation of B-centers is associated with creating a carbon vacancy and simultaneous occupation of an interstitial space by the dislocated carbon atom (Olivier et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Such interstitial carbon atoms aggregate to form a platelet (Woods G. S. 1986; Olivier et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows IR spectra of all the GIA standard reference diamonds of colour grades and saturation seen in the region from 1332 to 1150 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for F to M colour grades. For clarity each spectrum is offset by 1 unit on Y axis. It is well known that the intensity/absorbance of nitrogen aggregate peaks at 1282 and 1172 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e depends upon concentration of nitrogen (Woods et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). Absorbance in this region is strong because of high nitrogen concentration and characteristic peaks in this region were engulfed due to signal saturation.\u003c/p\u003e \u003cp\u003eBy inspecting the Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) it appears that peak area corresponding to 484 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is increasing with grade. However preliminary analysis w.r.t. to grades of the diamonds didn\u0026rsquo;t yield a good correlation and therefore this peak was not considered for further analysis.\u003c/p\u003e \u003cp\u003ePlatelet peaks are observed in the wavenumber region from 1360 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 1380 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. With increasing nitrogen concentration in diamonds, it is observed that the platelet peak position\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eshifted towards higher wavenumber. To visualise the platelet peak shift, relevant part of IR spectra in the region of 1350 to 1450 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, for diamonds of colour grade from E to M are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). It is seen from the Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) that the platelet peak position of all the measured diamonds are in the range from 1361 to 1370 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The platelet peak gets broadened for the diamonds from colour grade E to M and that is reflecting in the respective peak width (FWHM). From the measured IR spectra, values of the platelet peak position and FWHM were evaluated, and are given in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePeak position frequency and FWHM of platelet peak from measured IR spectra along with EPR measured unpaired nitrogen concentration {taken from (Lee et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)} for all the diamond standards.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSr.\u003c/p\u003e \u003cp\u003eNo.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eWeight\u003c/p\u003e \u003cp\u003e(Carats)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eColour Grade\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eType\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePeak position\u003c/p\u003e \u003cp\u003e(cm\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003eFWHM\u003c/p\u003e \u003cp\u003e(cm\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eN3\u0026thinsp;+\u0026thinsp;C- centre signal (Lee et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)\u003c/p\u003e \u003cp\u003e(ppm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.726\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIaA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1361.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e8.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0008047\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.780\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1364.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e10.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0013584\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.706\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eG\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1365.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e10.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0036994\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.718\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1365.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e10.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0061923\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.701\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1368.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e12.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0111028\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.709\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eJ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1370.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e13.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0180881\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.702\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1370.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e13.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0355232\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.721\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1366.20, 1362.79(split)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e14.68 (combined value)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.0581547\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.741\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa (saturated)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003e1367.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e13.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c9\" namest=\"c8\"\u003e \u003cp\u003e0.0711033\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFWHM has increased from grade E to M as shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. In the spectrum of grade L diamond, a doublet was observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b)) in the platelet region. Deconvolution of the platelet peak was done by using ORIGIN 9.5 programme for L grade. A sharp peak at 1366.20 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with an FWHM of 1.63 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a broad peak of 1362.79 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with an FWHM of 13.05 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were obtained. Sum of both the FWHMs is taken for the analysis and also given in the Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. However, it is not clear whether the platelet peaks for other diamonds are multiplets or not. Deconvolution of the platelet peaks for all IR spectra of the diamonds did not show any trend.\u003c/p\u003e \u003cp\u003eNitrogen concentration values were taken from Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) who estimated N concentration from their measurement of unpaired nitrogens by EPR and values are also given in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Although measured values are for unpaired electron containing nitrogen atoms, they reported a correspondence to total nitrogen present in the diamonds. This variation of N concentration could affect planar orientation within the platelet. Increasing nitrogen content in the diamonds could increase the peak position frequency of the platelet which might be the consequence of the stress it causes on the platelet. This stress could result in the fluctuation in vibrational energy affecting the shape and position of the observed peak. For larger concentration of N centres, electron density increases with increasing N atoms (Woods G. S. 1986). To understand this, we also examined variation of FWHM of the platelet peak as a function of nitrogen concentration in the diamonds. Olivier et al., (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) carried out monolayer imaging studies on type Ia diamonds and found that the concentration of nitrogen varied from 6 to 61% in a monolayer in the plane [100].\u003c/p\u003e \u003cp\u003eAs the main objective of this work is to arrive at correlations between colour grade of standards and observable parameter like platelet peak position or platelet peak width from IR spectra or absorption intensities from UV-Vis measurements, measured data were analysed as described in the following.\u003c/p\u003e \u003cp\u003eA scatter plot of platelet peak position versus the colour grade, giving arbitrary numerical values from 2 to 10 for the grades from E to M respectively, is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Similar arbitrary units for diamond colour grades were used by Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) for correlating EPR data with the colour grades of the diamond. Although the peak position (frequency) increases with respect to the grade, the shift is less pronounced for the grades H and L, and deviates from the trend. Woods G. S. (1986) had observed pronounced scattering in the plot of platelet peak position as a function of absorption coefficient for a large number of diamonds which is a deviation from a straight line relation given by Davies G., (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1977\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePresence of nitrogen inside the platelets through the formation of a C-N bond could also affect the vibrational energy (Woods G. S. 1986). It is well known that the vibration frequency is inversely proportional to the reduced mass of vibrating molecule and is directly proportional to the bond strength. Presence of C-N bond (bond strength\u0026thinsp;=\u0026thinsp;340 kJ/mol.) therefore might shift the characteristic band corresponding to platelet to higher value. Thus, it is possible that vibrational absorption peaks could shift to higher wavenumbers. This argument suggests that an increase in overall N concentration in the crystal might lead to a disorientation of the platelet structure in a way that creates stronger C-N bond within the platelet (Woods G. S. 1986; Sumiya et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). However, if this logic is applicable, then a monotonous increase could have been observed contrary to the present result and as well reported by Woods G. S., (1986).\u003c/p\u003e \u003cp\u003eTo examine for a possible correlation of platelet peak position frequency with the diamond colour grade, data were fitted to a polynomial giving arbitrary numerical values from 2 to 10 for the grades from E to M respectively and excluding outliers for grade L and M (ca. in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The fitted curves are also shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Our data are well represented by a second order equation (black colour line) as well as by a linear equation (red colour line). R\u003csup\u003e2\u003c/sup\u003e values obtained for both the fits are respectively, 0.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 for linear fit and 0.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 for second order fit. The R\u003csup\u003e2\u003c/sup\u003e values indicate that both the fits are \u0026ldquo;fit for the purpose\u0026rdquo; and could be used for evaluating the colour grade of the polished diamonds.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt is possible that the platelets could be undergoing a constant stress-strain phenomenon with respect to the host diamond crystal. Hence as the grade varies from E to M, increasing concentration of nitrogen might enhance the crystal stress encountered by the interplanar platelet defects. As a consequence, fluctuations of vibrational energy levels might be occurring (Sumiya et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) resulting in change in the peak width, as reported by Woods G. S., (1986). Our data on FWHM were analysed to examine for a possible relation between FWHM of the platelet peak and diamond colour grade.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e is a plot of FWHM of the platelet peak as a function of colour grade of the diamonds using arbitrary units (A.U.) like earlier. This Figure also contains best fitted lines. The FWHM values are increasing from E to M grades. From Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, it is clear that both the fits are good with R\u003csup\u003e2\u003c/sup\u003e values of 0.96\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 and 0.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 respectively for second order fit (black colour) and linear fit (red colour line).\u003c/p\u003e \u003cp\u003eThe fitted curves are used to evaluate grades for diamonds of different sizes. As given in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, IR spectra for 336 diamond test samples were measured to determine their grade using the correlations obtained in the present methodology. From the IR spectra, peak position and FWHM of the platelet peak were evaluated. Grades of the test samples were evaluated using platelet peak position in the equation obtained by using the linear fit (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). These values are compared with the grades determined using conventional method by GII graders. Giving numerical values to grades, the differences between GII grades and evaluated grades by the present spectrometric method are given in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. For 135 diamonds, variation is within one grade. For 101 diamonds variation is within 2 grades and for another 80 diamonds variation is within 3 grades whereas deviation is higher for(Lee et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) 14 diamonds. Grades of these 336 diamonds have also been evaluated using second order fit parameters and results obtained are given in column 4. Deviations for about 200 diamonds from both the sets of results (given in columns 3 \u0026amp; 4) are 2 or more grades and therefore this methodology needs to be evaluated further to consider for developing a colour grading methodology. Also to obtain better statistics a large number of test samples are to be measured.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDeviations of grades of test diamond samples determined by the present method compared to the grades determined by GII graders.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSr. No.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eColour grades deviation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003eNo. Samples\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eUsing 1st order\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eUsing 2nd order\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0 to \u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e135\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e141\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;1 to \u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e101\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e112\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;2 to \u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;3 \u0026lt;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSince presence of nitrogen gives colour tinge to diamonds, absorbance in the visible region (415 nm) was measured. An attempt is made to examine the possible correlation between platelet peak position and total N concentration taken from Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows a plot between platelet peak position obtained from IR spectra of GIA standards and nitrogen concentration from EPR measurements by Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Although data fit well to a second order equation, it does not explain why two different nitrogen concentrations have same platelet frequency. Thus, it gives two different grades and vice versa. This suggests that platelet peak position is not a good parameter for diamond colour grade determination.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows a fitted curve between the FWHM of the platelet peak positions as a function of nitrogen concentration (N3\u0026thinsp;+\u0026thinsp;C). The fit follows a polynomial given in Eq.\u0026nbsp;(1),\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$Y=-958 {X}^{2}+147.40 X+9.10 \\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere, Y is FWHM of the platelet and X is N concentration taken from Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Both the fits show nearly similar range of R\u003csup\u003e2\u003c/sup\u003e values of 0.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 and 0.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 respectively. Thus, it could be said that both FWHM and peak positions showed a reasonable correlation with the nitrogen concentration.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eUV-Vis NIR Measurements\u003c/h2\u003e \u003cp\u003eAs described earlier, in the process of transformation of \u0026lsquo;A\u0026rsquo; cluster into \u0026lsquo;B\u0026rsquo; cluster, it is possible to create a three nitrogen atom cluster around a vacancy known as N3 centre (Woods G. S. 1986; Olivier et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). This center absorbs blue light in the visible region giving a peak at 415 nm. Yellow tint of the diamond increases with increasing grade and it reflects in an increase in the intensity of the peak at 415 nm\u003c/p\u003e \u003cp\u003eWe adopted the nitrogen concentrations for GIA standards from Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). As described in the \u003cspan refid=\"Sec1\" class=\"InternalRef\"\u003eintroduction\u003c/span\u003e section, we thought of developing a simple method of colour grading through UV-Vis. measurements of diamonds. For that reason, we also analysed our samples with a UV-Vis. absorption spectrometer. As colour is the major criterion for deciding colour grade, UV-Vis. absorption spectral measurements were made. Absorption data were analysed to examine a plausible relation between absorption and colour grade. We recorded UV-Vis. absorption spectra for all the GIA colour standards at room temperature under similar experimental conditions. All the GIA standards have nearly same weight and RBC cut as given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. According to Beer-Lambert law, absorbance is directly proportional to the path length through which source light beam passes through the material. For RBC cut diamond, the longest path is along the girdle, and therefore diamond is placed on girdle position. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the UV-Vis absorption spectra of all GIA standards. Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), besides measuring concentration of single nitrogen atoms with unpaired electron by EPR spectroscopy, also obtained absorbance corresponding to N3 (415 nm) and C (477 nm) centre by UV-Vis spectroscopy.Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) established a good correlation between unpaired nitrogen concentration and colour of the diamond.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt is interesting to note that nitrogen concentration correlated well with the trends seen with UV-Vis (electronic transition) absorption data, common link being the colour grade. We explored to examine whether N3 Peak area correlates with the concentration of N3\u0026thinsp;+\u0026thinsp;C centre given by Lee et al., (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). The derived analytical function appears to suggest peak area of N3 centre might be a good parameter to determine a colour grade.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows a scatter plot of N3 (415 nm) peak area of GIA standards and test sample set \u0026ndash; 5 with respective colour grades. The plot for Sample set-5 (red filled circles) shows nearly similar trend as that of GIA standards (Black squares) from higher to lower colour grades. The peak areas of N3 centre also show similar trend with continuous increase in the peak area with increasing colour grade except for grade K.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis is an encouraging sign to develop a methodology that could be converted to fabricate a machine to determine the colour grades of polished diamonds by measuring the absorbance in N3 window using visible/colour spectrometer. Total absorbance in the visible region appears to be another reliable parameter to correlate colour grading of a diamond. However, we have to test more samples for further examining this methodology.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003ePlatelet peak position and its FWHM from measured IR spectra of GIA standard diamonds were evaluated. Using the fit parameters obtained from plots between peak position and colour grade of the standards, colour grades for 336 diamonds of different sizes were evaluated. Although colour grades between thus computed values by present method and determined by GII graders are in reasonable agreement, deviation for 195 diamonds are two or more grades. Therefore, this methodology cannot be accepted as reliable. On the other hand, on other hand a good correlation between platelet position measured by us and N3\u0026thinsp;+\u0026thinsp;C from literature was obtained for the same set of diamonds but the double values for the same platelet frequency does not encourage to adopt this methodology. Further, visible absorbance of GII standards gave a good correlation with the grades. Measured absorbance of a set of diamonds of similar size, compared well with the trends obtained with GIA standards. We are hopeful that a methodology could be developed for colour grading of diamonds using measurement of absorbance in the visible region that would lead to fabricate a colour grading machine by incorporating a simple visible absorption spectrometer.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization, R.A.; B.J. and M.G.; methodology, R.A.; B.J. and M.G.; software, R.A.; validation, R.A.; B.J.; M.G. and A.V.R.; formal analysis, R.A.; investigation, R.A.; and A.V.R.; resources, R.A.; A.G.; and A.V.R.; data curation, R.A.; and A.G.; writing\u0026mdash;original draft preparation, R.A.; A.G.; M.G.; S.M.; and A.V.R.; writing\u0026mdash;review and editing, R.A.; A.G.; M.G.; S.M.; H.B.; and A.V.R. All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere are no conflicts of interest to declare.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAuthors thank Dr. M.D. Sastry former Head, R\u0026amp;D, GII for initiating this work and Mr. Yogesh Salunkhe for help in characterizing the diamonds using confocal Raman microscopy details of which are not added in this manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBouman M, Anthonis A, Chapman J, et al (2018) The Effect of Blue Fluorescence on the Colour Appearance of Round-Brilliant-Cut Diamonds. The Journal of Gemmology 36:298\u0026ndash;315. https://doi.org/10.15506/JoG.2018.36.4.298\u003c/li\u003e\n\u003cli\u003eBoyd SR, Kiflawi I, Woods GS (1994) The relationship between infrared absorption and the A defect concentration in diamond. Philosophical Magazine B 69:1149\u0026ndash;1153. https://doi.org/10.1080/01418639408240185\u003c/li\u003e\n\u003cli\u003eBreeding CM, Eaton‐Maga\u0026ntilde;a S (2019) Fluorescence of Natural and Synthetic Gem Diamond: Mechanism and Applications. In: Encyclopedia of Analytical Chemistry. Wiley, pp 1\u0026ndash;26\u003c/li\u003e\n\u003cli\u003eBreeding CM, Eaton-Magana S, Shigley JE (2020) Naturally Colored Yellow and Orange Gem Diamonds: The Nitrogen Factor. Gems \u0026amp; Gemology 56:194\u0026ndash;219. https://doi.org/10.5741/GEMS.56.2.194\u003c/li\u003e\n\u003cli\u003eBreeding CM, Shigley JE (2009) The \u0026ldquo;Type\u0026rdquo; Classification System of Diamonds and Its Importance in Gemology. Gems \u0026amp; Gemology 45:96\u0026ndash;111. https://doi.org/10.5741/GEMS.45.2.96\u003c/li\u003e\n\u003cli\u003eDavies G. (1977) Chemistry and Physics of Carbon. Dekker, New York, USA\u003c/li\u003e\n\u003cli\u003eD\u0026rsquo;Haenens-Johansson UFS, Soe Moe K, Johnson P, et al (2014) Near-Colorless HPHT Synthetic Diamonds from AOTC Group. Gems \u0026amp; Gemology 50:30\u0026ndash;45. https://doi.org/10.5741/GEMS.50.1.30\u003c/li\u003e\n\u003cli\u003eEaton-Magana S, Ardon T, Breeding CM, Shigley JE (2020) Natural-Color D-to-Z Diamonds: A Crystal-Clear Perspective. Gems \u0026amp; Gemology 56:318\u0026ndash;335. https://doi.org/10.5741/GEMS.56.3.335\u003c/li\u003e\n\u003cli\u003eEaton-Magana S, Ardon T, Breeding CM, Shigley JE (2019) Natural-Color Fancy White and Fancy Black Diamonds: Where Color and Clarity Converge. Gems \u0026amp; Gemology 55:. https://doi.org/10.5741/GEMS.55.3.320\u003c/li\u003e\n\u003cli\u003eEaton-Maga\u0026ntilde;a S, Ardon T, Smith K V., et al (2019) Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors. Gems \u0026amp; Gemology. https://doi.org/10.5741/GEMS.54.4.352\u003c/li\u003e\n\u003cli\u003eEaton-Magana S, Breeding CM (2016) An Introduction to Photoluminescence Spectroscopy for Diamond and its Applications in Gemology. Gems \u0026amp; Gemology 52:2\u0026ndash;17. https://doi.org/10.5741/GEMS.52.1.2\u003c/li\u003e\n\u003cli\u003eEaton-Maga\u0026ntilde;a S, Breeding CM, Shigley JE (2018) Natural-color blue, gray, and violet diamonds: Allure of the deep. Gems and Gemology 54:112\u0026ndash;131. https://doi.org/10.5741/GEMS.54.2.112_i\u003c/li\u003e\n\u003cli\u003eEvans T., Qi Z. (1982) The kinetics of the aggregation of nitrogen atoms in diamond. Proceedings of the Royal Society of London A Mathematical and Physical Sciences 381:159\u0026ndash;178. https://doi.org/10.1098/rspa.1982.0063\u003c/li\u003e\n\u003cli\u003eFridrichov\u0026aacute; J, Bacik P, \u0026Scaron;koda R, Antal P (2015) Use of spectroscopic methods for determination of diamond origin and treatment. Acta Geologica Slovaca 7:11\u0026ndash;18\u003c/li\u003e\n\u003cli\u003eGarc\u0026iacute;a-Torres J.M. (2018) Implementing a deep learning algorithm for diamond classification\u003c/li\u003e\n\u003cli\u003eGreen BL, Collins AT, Breeding CM (2022) Diamond Spectroscopy, Defect Centers, Color, and Treatments. Rev Mineral Geochem 88:637\u0026ndash;688. https://doi.org/10.2138/rmg.2022.88.12\u003c/li\u003e\n\u003cli\u003eHainschwang T, Notari F, Fritsch E, Massi L (2006) Natural, untreated diamonds showing the A, B and C infrared absorptions (\u0026ldquo;ABC diamonds\u0026rdquo;), and the H2 absorption. Diam Relat Mater 15:1555\u0026ndash;1564. https://doi.org/10.1016/j.diamond.2005.12.029\u003c/li\u003e\n\u003cli\u003eISO 24016 (2020) Jewellery and precious metals grading polished diamonds terminology, classification and test methods. In: ISO 24016. https://standards.iteh.ai/catalog/standards/iso/d625a4fd-e4d4-48f2-b136-4bd4d88303de/iso-24016-2020. Accessed 18 Sep 2020\u003c/li\u003e\n\u003cli\u003eKing J.M., Shigley J.E., Gelb T.H., et al (2005) Unusually large novelty cut. Gems \u0026amp; Gemmology 88\u0026ndash;111\u003c/li\u003e\n\u003cli\u003eLee CWY, Cheng J, Yiu YC, et al (2020) Correlation between EPR spectra and coloration of natural diamonds. 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New J Phys 13:035024. https://doi.org/10.1088/1367-2630/13/3/035024\u003c/li\u003e\n\u003cli\u003eRabinowitz Y, Etinger A, Litvak B, et al (2021a) Millimeter wave spectroscopy for evaluating diamond color grades. Diam Relat Mater 116:108386. https://doi.org/10.1016/j.diamond.2021.108386\u003c/li\u003e\n\u003cli\u003eRabinowitz Y, Etinger A, Yahalom A, et al (2021b) Microwave Spectroscopy as a Potential Tool for Color Grading Diamonds. Energies (Basel) 14:3507. https://doi.org/10.3390/en14123507\u003c/li\u003e\n\u003cli\u003eSumiya H, Sakano F, Tatsumi N (2023) Distribution of internal strain and fracture strength in various single-crystal diamonds. Diam Relat Mater 134:109781. https://doi.org/10.1016/j.diamond.2023.109781\u003c/li\u003e\n\u003cli\u003eWoods G. S. (1986) Platelets and the infrared absorption of type Ia diamonds. Proceedings of the Royal Society of London A Mathematical and Physical Sciences 407:219\u0026ndash;238. https://doi.org/10.1098/rspa.1986.0094\u003c/li\u003e\n\u003cli\u003eWoods GS, Purser GC, Mtimkulu ASS, Collins AT (1990) The nitrogen content of type Ia natural diamonds. Journal of Physics and Chemistry of Solids 51:1191\u0026ndash;1197. https://doi.org/10.1016/0022-3697(90)90101-K\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Vacancy in diamonds, Platelets frequency, IR spectrum, UV-Vis-NIR spectrum, N3 centre, Nitrogen Aggregate, Full Width at Half Maximum, Fitting of data, Colour grading of diamonds.","lastPublishedDoi":"10.21203/rs.3.rs-4593034/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4593034/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eColour grading of diamonds is determined routinely by experienced graders, and grades range from D to Z, with D being pure white and valuable. Efforts by researchers are focused to develop an instrumental method for bias-free grading. Presence of nitrogen in type Ia diamonds results in yellow tint and its intensity increases with increase in nitrogen content (grades from D to Z). In the present work electronic and vibrational spectra of diamond standards of Gemological Institute of America (GIA) were measured to develop and standardize the methodology by utilizing nitrogen based vacancies. Data on platelet peak position and its width were analysed for a possible correlation with the colour grade. Platelet peak position data obtained from IR spectra gave a good correlation with colour grade upto grade L. For the grades E to M, data fitted well to both quadratic (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02) and linear equations (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01). Using peak position fit, colour grades of 336 diamonds were evaluated and compared with visual grades that were determined by the graders of Gemmological Institute of India (GII) and results are discussed. Absorbance of N3 Peak (415 nm) in the visible region also gave a good correlation with colour grading. Colour grades of diamonds referred as test samples were determined using UV data and the results are encouraging. Therefore, it appears feasible to develop an instrumental methodology for colour grading of type-Ia diamonds based on only absorption measurements in the visible region.\u003c/p\u003e","manuscriptTitle":"Spectroscopic Evaluation of Colour Grading of Type Ia Diamonds†.","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-11 16:10:53","doi":"10.21203/rs.3.rs-4593034/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"476e34c1-e297-4077-ae61-7703fddd34e2","owner":[],"postedDate":"July 11th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-07-31T21:00:41+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-11 16:10:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4593034","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4593034","identity":"rs-4593034","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}