Fancy Black NPD Synthetic Diamond

IMG - Fancy Black 159262 636x358
Figure 1. This 0.9 ct marquise proved to be a synthetic nano-polycrystalline diamond (NPD), the first one detected by a gemological laboratory. Photo by Jian Xin (Jae) Liao/GIA.
As reported previously in Gems & Gemology, nano-polycrystalline diamond (NPD) growth technology is one of the most significant new developments in diamond synthesis (see E.A. Skalwold, “Nano-polycrystalline diamond sphere: A gemologist’s perspective,” Summer 2012 G&G, pp. 128–131). The sphere’s material is completely transparent and comparable to natural diamond, while tougher than natural diamond or previous synthetic diamonds. NPD could pose a challenge to the natural diamond industry once these goods, particularly higher-quality transparent samples, enter the consumer market (E.A. Skalwold et al., "Characterization of a synthetic nano-polycrystalline diamond gemstone," Fall 2012 G&G, pp.188–192).

Recently, the New York laboratory tested a small, black marquise-cut stone for identification and color origin. At first glance, the 0.9 ct translucent specimen (figure 1) resembled a typical black diamond submitted to the lab for identification and color origin; one must be careful to separate such stones from synthetic moissanite. This marquise, however, revealed unusual properties strikingly similar to those of the NPD synthetic diamonds reported on by Skalwold, as well as other previously tested samples. These previously studied specimens were transparent with a yellow to brown range of color. 

IMG - Fancy Black 159261 636x509
Figure 2. Micrograph images of the heavily included synthetic diamond showed an abundance of graphite inclusions and a murky yellowish matrix. Photomicrographs by Paul Johnson/GIA, fields of view (clockwise): 1.5 mm, 3.1 mm, 1.5 mm, and 4.1 mm.
Unlike natural diamond, the stone in question was heavily included with graphite crystals. The matrix hosting the inclusions had a murky yellowish color (figure 2). Using diffuse reflectance, we obtained a mid-infrared absorbance spectrum strikingly similar to that of the NPD synthetic diamonds reported in the aforementioned article. We observed the diagnostic diamond absorption peaks at approximately 2000 cm–1 and absorption in the one-phonon region, possibly due to nitrogen impurity. Peaks at approximately 3727 and 3611 cm–1 were also observed, as in some previously tested NPD samples (figure 3). Raman spectroscopy is a common and useful technique in identifying a gem diamond. Due to the nano-sizes of the crystals that compose the aggregated sample, a Raman shift is usually not detectable in NPD using visible-light laser excitations. The same feature was observed in this specimen.

IMG - Fancy Black NPD spectra 636x358
Figure 3. The mid-infrared spectrum of the 0.9 ct black marquise-cut synthetic NPD (in red) displayed peaks at approximately 3727 and 3611 cm–1, which have been observed in some other NPD samples. The mid-IR spectrum in 2012 by Skalwold et al. (in blue) is provided for comparison.
Further analysis using the DiamondView instrument revealed a fluorescence pattern and structure that was, again, very similar to earlier NPD samples (figure 4). Raman analysis showed a broad band at approximately 1350 cm–1 and a weak band at about 1580 cm–1; these are assigned as D- and G-bands, respectively (S. Odake et al., “Pulsed laser processing of nano-polycrystalline diamond: A comparative study with single crystal diamond,” Diamond and Related Materials, 2009, Vol. 18, pp. 877–880). Odake reported the two bands in NPD samples grown from high-purity graphite at a temperature of 2600K and a pressure of 15 GPa; both are caused by nanocrystalline graphite or amorphous carbon. Unlike Odake’s samples, which showed G- and D-bands of equal intensity, ours showed a strong D-band with a weak G-band. We measured Raman spectra at many different locations, but did not observe the diamond Raman band at 1332 cm–1, an absence Odake also reported. This suggests the presence of sp2-bonded carbons instead of sp3-bonded carbons.

IMG - Fancy Black 159260 636x444
Figure 4. DiamondView images of the synthetic NPD exhibited a distinct fluorescence pattern. Images by Paul Johnson/GIA.
Testing and observations revealed the diamond to be an NPD synthetic diamond, the first detected by a gemological laboratory.

Paul Johnson and Kyaw Soe Moe are researchers at GIA's New York laboratory.