Lab Notes Gems & Gemology, Fall 2014, Vol. 50, No. 3

Two Fancy Dark Gray CVD Synthetic Diamonds

1.50 克拉的花式深灰色 CVD 合成钻石
Figure 1. This 1.50 ct Fancy Dark gray CVD synthetic diamond was unusual for its combination of color grade and large size. Photo by Jian Xin (Jae) Liao.
Recently, two colored synthetic diamonds examined in the Carlsbad laboratory were determined to have been grown by the chemical vapor deposition (CVD) method. Both received an unusual color grade of Fancy Dark gray. GIA has graded other gray CVD synthetics before, but within the Fancy Light to Fancy range; it is rare for a synthetic to receive a Fancy Dark color grade. The few Fancy Dark synthetic diamonds submitted in the past have all been HPHT synthetics (most of them Fancy Dark reddish brown) with post-growth treatment that created NV centers. The two gray CVD specimens were also noted for their size—0.80 and 1.50 ct. The latter is shown in figure 1.
In addition to their color grade, both showed similar spectra. Neither sample’s UV-visible-NIR spectrum displayed any distinct absorption peaks. The mid-IR spectra identified them as type IIa diamond but did not contain features that confirmed synthetic origin. Their photoluminescence (PL) spectra had nearly identical features, suggesting that they originated from similar manufacturing processes. Both PL spectra featured a large silicon-vacancy doublet at 737 nm, a strong indicator of CVD growth (though natural diamonds will occasionally show a weak silicon peak along with other natural features); this was confirmed by the DiamondView images. The 1.50 ct specimen showed light blue fluorescence and striations commonly observed in CVD synthetics (figure 2, left) (P. Martineau et al., “Identification of synthetic diamond grown using chemical vapor deposition (CVD),” Spring 2004 G&G, pp. 1–25). The 0.80 ct sample offered a more unusual DiamondView image, with yellow fluorescence interspersed with regions of purple mottling. The purple mottling is seen occasionally in CVD synthetics, but usually combined with a background of pink fluorescence (W. Wang et al., “Latest-generation CVD-grown synthetic diamonds from Apollo Diamond Inc.,” Winter 2007 G&G, pp. 294–312) instead of the yellow fluorescence seen here. Some regions of the 0.80 ct synthetic had natural-looking fluorescence features (figure 2, right), but a thorough examination of the DiamondView fluorescence in conjunction with the silicon peak unam­biguously identified it as CVD.

CVD Synthetics DiamondView
Figure 2. Left: In the DiamondView, the 1.50 ct sample showed the familiar color and striations of CVD synthetics. Right: The 0.80 ct specimen displayed a combination of natural-appearing features in some regions (as shown) and CVD-related striations and purple mottling in others. Images by Sally Magaña.
The DiamondView images of both synthetics also demonstrated blue phosphorescence, which is often associated with boron impurities. No known boron-related peaks were detected in the PL or mid-IR spectra, so the boron concentration would be below the level of detection by FTIR spectroscopy. Boron-related peaks seen in natural and treated type IIb diamonds often are not detected in known IIb HPHT- and CVD-grown synthetics. It is unclear whether the low boron concentration was deliberately introduced to improve growth rate and quality (S. Eaton et al., “Diamond growth in the presence of boron and sulfur,” Diamond and Related Materials, Vol. 12, 2003, pp. 1627–1632). Boron-containing diamonds with low color saturation often appear gray, making this element the likely cause of color in both of these CVD synthetic diamonds. 

Troy Ardon is a staff gemologist and diamond color origin specialist, and Sally Eaton-Magaña is a research scientist, at GIA’s laboratory in Carlsbad, California.