Summer 2002, Volume 38, Issue 2

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Featured Lab Note: Another Commercial U.S. Facility Offers HPHT Annealing
Wuyi Wang

It recently came to our attention that another U.S. facility is offering high-pressure, high-temperature (HPHT) annealing of diamonds to improve their color. Phoenix Crystal Corp. of Ann Arbor, Michigan, an industrial supplier of diamond abrasives and cutting tools, is commercially treating gem diamonds using a standard belt-type press. Dr. Robert Frushour, president of Phoenix Crystal Corp., has informed us that his facility has both decolorized brown type IIa diamonds and produced a range of colors in type I diamonds. The figure shows a 1.07 ct type Ia diamond before and after HPHT annealing at this facility. Similar to other samples with comparable properties, this one illustrates the predictable color enhancement from brown to green-yellow (see, e.g., I. M. Reinitz et al., "Identification of HPHT-treated yellow to green diamonds," Summer 2000 Gems & Gemology, pp. 128-137). Figure 3 [not shown online] shows the mid- and near-infrared absorption spectra of the 1.07 ct diamond before and after treatment. The presence of the H2 absorption feature at 10,126 cm-1 (approximately 987 nm) in the near-infrared spectrum is indicative of similarly annealed type I diamonds, as are the graphitized "feathers" seen with a gemological microscope. Some variations were also observed in the mid-infrared range. According to Dr. Frushour, the normal annealing temperature used at their facility is generally above 1800°C, with pressure in the range of 5 to 6 GPa. The diamonds annealed range from under 1 ct to over 10 ct. Phoenix Crystal Corp. is not marketing diamonds processed at its facility, but is offering the HPHT service to the trade. We do not know the exact number of commercial HPHT annealing facilities in the U.S., but we suspect that it is very small. Wuyi Wang (Caption) Photo by Elizabeth Schrader

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