Separation of Black Diamond from NPD Synthetic Diamond
In two recent Lab Notes, we reported on a new type of synthetic diamond: nano-polycrystalline synthetic diamond, known as NPD (Spring 2014, pp. 69–71; Winter 2014, pp. 300–301). Submitted for identification in April 2016 was a 0.70 ct pear-shaped Fancy black diamond (figure 1). The diamond’s infrared absorption spectrum was strikingly similar to that of the two NPD identified specimens mentioned above. It displayed very similar absorption peaks in the one-phonon region (figure 2), which can probably be attributed to nitrogen.
Microscopic examination revealed an abundance of dark graphitized crystal and fracture inclusions, features often associated with black gem-quality diamonds but not unlike those observed in the NPD samples (figure 3). The challenge for gem laboratories, then, is how to separate black NPD synthetic diamonds from their natural black diamond counterparts.
DiamondView imaging offers a quick and definitive solution to this problem. NPD synthetic diamond has a distinct fluorescence pattern and structure that are obvious in the DiamondView images (figure 4). This technique can provide an instant positive identification for NPD synthetic diamond, which can be supported with further testing.
The 0.70 ct pear-shaped diamond was issued a report with a Fancy black color grade and a natural origin of color.