Spring 2011

A Very Large Colorless HPHT-Treated Diamond

Over the past year, the laboratory has observed an influx of large HPHT-treated diamonds, both type I and type II (see Lab Notes: Spring 2010, pp. 50–51; Winter 2010, p. 298; and the previous entry in this issue). This trend has continued in 2011.

Among the HPHT-annealed diamonds we have identified recently, one was exceptional. This cushion-cut diamond weighed 38.59 ct (24.37 x 18.69 x 10.22 mm) and was graded F color with a clarity of VVS₁. Microscopic examination revealed internal graining in a few orientations, and strong internal strain was seen with crossed polarizers. Such features are common in HPHT-treated type IIa diamonds (T. M. Moses et al., “Observations on GE-processed diamonds: A photographic record,” Fall 1999 G&G, pp. 14–22).

The IR absorption spectrum re­vealed a typical type IIa diamond, with no detectable nitrogen- or hydrogen-related features. The UV-Vis spectrum also showed no isolated nitrogen-related absorptions. However, photoluminescence spectroscopy collected at liquid-nitrogen temperature with laser excitations of varying wavelength confirmed HPHT annealing, consistent with the graining and strain patterns seen with the microscope.

This is one of the largest HPHT-treated diamonds the GIA Laboratory has ever identified. Treating diamonds this size is risky, and we suspect that not all facilities have the equipment to anneal them, as the capsule normally used in HPHT presses will not accommodate such a large stone. It is also possible that such diamonds take longer to process.

Gem diamonds of this size and quality are extremely rare. The increasing number of large HPHT-treated diamonds seen in the laboratory underscores the importance of proper disclosure and reliable gem identification.

Wuyi Wang and Tom Moses