Very Large Irradiated Yellow Diamond
This emerald-cut stone weighed 22.27 ct and was color graded as Fancy Vivid yellow (figure 1). Faint color concentration was observed along the culet. The diamond showed medium yellow-green, slightly chalky fluorescence to long-wave UV and weak orange fluorescence to short-wave UV. No obvious internal features were observed. Its infrared absorption spectrum showed high concentrations of nitrogen and weak absorptions from defects H1b (4935 cm–1) and H1c (5165 cm–1). A weak hydrogen-related absorption at 3107 cm–1 was also detected. In the Vis-NIR region, the absorption spectrum collected at liquid-nitrogen temperature showed strong absorptions from the N3, H4, H3, and 595 nm optical centers (figure 2). These spectroscopic and gemological features demonstrated that the diamond had been artificially irradiated and annealed to introduce additional absorptions from the H3/H4 defects, enhancing its yellow color. This Fancy Vivid yellow color would have been much lighter and less saturated before the treatment.
Artificially irradiated diamonds of this size and attractive color are rarely examined in gem laboratories.