Large Green and Yellow Diamonds Colored by Nickel Impurities
Diamonds colored by nickel impurities were previously thought to only occur in lower carat weights and with weaker color saturations. However, the Carlsbad laboratory recently received several large examples with strongly saturated color. Among the stones examined, a Fancy Intense green-yellow cushion weighing 5.02 ct, a Fancy yellow-green pear weighing 5.03 ct, and a Fancy Intense green-yellow pear weighing 5.06 ct were the most notable (figure 1).
Gemologically, these diamonds were mostly free of inclusions except for faint internal graining and small reflective platelets. Along with natural inclusions, FTIR spectra showed that all three stones were type IaA, confirming that these are naturally mined diamonds and not of synthetic origin. Vis-NIR spectra revealed an asymmetrical absorption band around 670 nm and a notable peak at 883 nm for all three (figure 2). These features in the UV-Vis-NIR strongly suggest that the color is caused by the presence of nickel-related defects.
Previously, the largest and most intensely colored gem diamonds reported to have been colored by nickel defects were a 2.54 ct Fancy Light greenish yellow diamond (see Fall 2013 Lab Notes, pp. 173–174) and a 2.81 ct Fancy Intense yellowish green diamond (W. Wang et al., “Natural type Ia diamond with green-yellow color due to Ni-related defects,” Fall 2007 G&G, pp. 240–243).
The stones described in the present report are nearly twice as large and have significantly more saturated hues. Therefore, it appears that nickel-colored diamonds can occur over a wider range of sizes and color saturations than previously suspected.
