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 (R2 = 0.92 ± 0.02) and linear equations (R2 = 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.