Winter 2008

Featured Lab Note: HPHT-Treated CVD Synthetic Diamond Submitted for Dossier Grading

Gems & Gemology has reported on synthetic diamonds grown by the chemical vapor deposition (CVD) method for several years (e.g., W. Wang et al., “Gem-quality synthetic diamonds grown by a chemical vapor deposition [CVD] method,” Winter 2003, pp. 268–283; P. M. Martineau et al., “Identification of synthetic diamond grown using chemical vapor deposition [CVD],” Spring 2004, pp. 2–25; and W. Wang et al., “Latest-generation CVD-grown synthetic diamonds from Apollo Diamond Inc.,” Winter 2007, pp. 294–312). CVD synthetic diamonds have been submitted to the GIA lab (e.g., Lab Notes: Spring 2008, pp. 67–69, and Summer 2008, pp. 158–159), but those samples were as-grown, even though high-pressure, high-temperature (HPHT) treatment of CVD synthetics is well known.

The 0.21 ct round brilliant in figure 1 was recently submitted for Diamond Dossier grading. Standard testing identified it as a CVD synthetic. It was graded near colorless (GIA does not use letter grades on synthetic diamond reports) and VVS, with the clarity grade based on pinpoint inclusions. Between crossed polarizers, it exhibited weak birefringence in shades of gray (figure 2). In comparison, the as-grown sample described in the Spring 2008 Lab Note showed strong birefringence with high-order interference colors, consistent with the samples examined by Wang et al. (2007).

The sample was inert to long-wave UV radiation but fluoresced weak yellow to short-wave UV. In the DiamondView, it fluoresced blue-green, with yellow-green striations (figure 3), and exhibited weak blue phosphorescence. The fluorescence color was similar to the green luminescence of HPHT-treated nitrogen-doped CVD synthetic diamonds reported by Martineau et al. (2004), and it contrasted distinctly with the orangy pink to orangy red hues of the as-grown CVD synthetics described in the previous Lab Notes. The striations are growth phenomena, and are typical of CVD synthetic diamonds (see, e.g., Martineau et al., 2004).

The mid-IR spectra established that the sample was type IIa. Neither of the hydrogen-related peaks at 3123 and 3107 cm−1 was detected. Martineau et al. (2004) stated that the former would be removed by HPHT treatment, while the latter could appear after HPHT treatment. A weak peak was observed at 1332 cm−1, consistent with the near-colorless samples described by Wang et al. (2007).

Photoluminescence spectra collected at liquid-nitrogen temperature (~77 K) with 488 and 514.5 nm laser excitation exhibited a very large peak doublet at 736.6/736.9 nm, due to the silicon-vacancy (Si-V) defect. Correspondingly, a very small doublet was recorded in the UV-visible spectrum at ~737 nm; the spectrum was otherwise featureless but with absorption rising toward the blue region, as is typical of a near-colorless type IIa diamond.

The Si-V defect was originally considered indicative of CVD synthetic diamonds; however, recent work has shown that the defect also exists in some natural colorless and near-colorless diamonds (C. M. Breeding and W. Wang, “Occurrence of the Si-V defect center in natural colorless gem diamonds,” Diamond and Related Materials, Vol. 17, 2008, pp. 1335–1344). Large peaks were observed at 575.0 and 637.0 nm (the zero-phonon lines [ZPLs] of the nitrogen-vacancy centers [N-V]0 and [N-V]−, respectively), consistent with CVD synthetic diamond (see, e.g., Wang et al., 2003). In contrast to the as-grown CVD synthetic diamonds documented in the two earlier Lab Notes, the PL spectra for this diamond did not exhibit a 596.5/597.0 nm doublet. Again, this is consistent with the HPHT-treated CVD synthetic diamonds discussed by Martineau et al. (2004). The 488 nm spectrum did display a large peak at 503.1 nm—the H3 ZPL—associated with nitrogen. This is also consistent with the results of Martineau et al. (2004), and contrasts with the relative lack of H3 peaks in the near-colorless as-grown CVD synthetic diamond samples investigated by Wang et al. (2007). It appears that the H3 defect is introduced during HPHT annealing.

We have seen very few HPHT-treated CVD-grown synthetic diamonds in the laboratory, but the criteria discussed above allowed us to successfully identify this sample.

Karen M. Chadwick