Chromaticity Study of Yellow HTHP Lab-Grown Diamonds Based on Spectroscopy

In recent years, lab-grown diamonds have become more popular in both domestic and international markets for their rich color palette. Research on yellow lab-grown diamonds has primarily focused on spectroscopic and defect characteristics currently, while the study has largely focused on nitrogen content and related color-causing mechanisms, such as NV series defects. However, the relationship between nitrogen content and defects and color is limited. In this study, eight lab-grown diamonds with varying yellow shades were selected as samples to be studied by photoluminescence spectra, infrared spectra, Raman spectra, and colorimetry testing. Based on the colorimetric parameters L*, a*, and b*, the standard formula for the yellowness index, the intensities of the NV0 and NV− peaks in the photoluminescence spectra and the absorptivity in the infrared spectra, the hue angle h, the yellowness index YI E313, the concentration ratio of NV− defect in NV color centers R, and the nitrogen content NC were calculated. Results indicate that characteristic peaks of NV series defects as a specific photoluminescence signature, notably the absence of [Si-V]− defect, demonstrate that the samples are high-temperature, high-pressure diamonds derived from graphite that underwent post-growth irradiation. The specific infrared signature indicates that the type of samples is type Ib, attributed to isolated nitrogen (C aggregate). The intrinsic peak of diamond is detected in Raman spectra, with symmetric stretching vibrations of C and N and the ‘D’ peak of graphite is detected as well. Meanwhile, the yellowness index shows a negative correlation with hue angle, a positive correlation with concentration ratio, and a positive linear correlation with nitrogen content, the equation y = 0.17x + 124.40. The yellowness index is divided into three levels: 70–80, 80–90, and 90–100. The yellow hue of samples is light between 70–80, intense between 80–90, and deep between 90–100.