Lab Notes Gems & Gemology, Summer 2016, Vol. 52, No. 2

Separation of Black Diamond from NPD Synthetic Diamond

Fancy black pear-shaped diamond.
Figure 1. The 0.70 ct Fancy black pear-shaped diamond in the left photo closely resembled two NPD synthetic diamonds submitted earlier (the 1.51 ct round and 0.9 ct marquise, right). Photos by Jian Xin (Jae) Liao.

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.

Unusual broad peaks in IR spectra.
Figure 2. IR spectra of the three black samples (offset for clarity) show unusual broad peaks in the 760–1500 cm–1 region (highlighted in gray): 0.70 ct natural black diamond (A), 1.51 ct NPD (B), and 0.90 ct NPD (C).

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.

Inclusions in natural and NPD diamond.
Figure 3. Natural inclusions in black diamond (left) are compared with various inclusions previously observed in NPD synthetic diamond (center and right). Photos by Paul Johnson; field of view 6.24 mm (left) and 1.76 mm (center and right).

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.

DiamondView of fluorescence in natural and NPD synthetic.
Figure 4. These DiamondView images show the fluorescence pattern and color for natural diamond (left and center) and NPD synthetic diamond (right). Photos by Paul Johnson.

The 0.70 ct pear-shaped diamond was issued a report with a Fancy black color grade and a natural origin of color.

Paul Johnson is a supervisor of diamond advanced testing, and Kyaw Soe Moe is a research associate, at GIA in New York.