Lab Notes Gems & Gemology, Spring 2022, Vol. 58, No. 1

Large Green and Yellow Diamonds Colored by Nickel Impurities

Nickel-related impurities are responsible for the strong color saturations in these diamonds.
Figure 1. Large gem diamonds with strong color saturations resulting from nickel-related impurities. Top: 5.02 ct Fancy Intense green-yellow cushion. Bottom left: 5.03 ct Fancy yellow-green pear. Bottom right: 5.06 ct Fancy Intense green-yellow pear. Photos by Diego Sanchez.

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).

Vis-NIR spectra indicate that nickel is the main cause of color in these diamonds.
Figure 2. Vis-NIR spectra collected from each diamond show a strong asymmetrical absorption band around 670 nm along with a distinct peak at 883 nm. These defects together, with the lack of any other color-causing defects, indicate that nickel is the main cause of color.

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.

Jamie Price is a staff gemologist at GIA in Carlsbad, California.