Just an Old Lump of Coal?

By Gary Roskin, G.G., FGA, Senior Editor -- JCK Online, 1/1/2002 2:00:00 AM

SSEF, the Swiss Gemmological Institute, recently changed its certification of black diamond. According to lab director Prof. Henry Hänni, large quantities of black diamonds in all sizes are being created by cooking ordinary crushing bort at high temperatures. The black bort is then cut and polished for use in trendy black diamond jewelry.

Heated black diamond appears similar to naturally colored black diamond. Rahul and Sunil Karnavat of Lotus Diagems in New York, specialists in irradiated blacks and other color-enhanced diamonds, sent JCK a handful of the heat-treated material, noting that their price tags are almost 20% of a natural black diamond's cost, and about 50% of the price of an irradiated black diamond.

"It's not that these diamonds are not worth the money," says Rahul Karnavat. "It's that most people would not know that they are buying a heated piece of bort. They think they are buying natural black diamond, or gem diamond that is heated to turn black."

The price of the heated material should be, and usually is, lower. "Our goal is not to say that heated black is not good," says Karnavat. "Our goal is to educate people about what they are buying, and then it is their choice."

Dr. George Rossman, professor of mineralogy at CalTech in Pasadena, Calif., has examined heat-treated black diamonds. "We did optical examination and Raman spectra and found that the diamonds were mostly diamond, but had 'fractures' within the diamond that had been [changed into graphite]." Rossman says the blackened graphite fractures were big enough and extended far enough to make the entire diamond appear black. The Gemological Institute of America notes that close-up visual observation of heat-treated blacks reveals graphite lining the walls of fractures and cavities.

But according to SSEF, ordinary gemological procedures are not sufficient to distinguish natural from heat-treated blacks. Consequently, early last year SSEF put its identification process on hold. According to Hänni, the laboratory now has a method by which it can make such a determination. However, details of this identification process have not been revealed.

Karnavat has a simple—and destructive—method for determining treatment. By soaking the diamond overnight in an acid (for example, nitric acid or any other strong oxidant that diamond cutters normally use to clean newly polished or laser-drilled stones), the black graphite is turned white. "The color will bleach, and the diamond will lose weight because its surface starts crumbling when subjected to heated nitric acid," says Karnavat. Acid cleaning, of course, leaves an off-color diamond, usually of industrial quality.

Prices for natural black diamonds should run close to $500 per carat, while irradiated blacks sell for approximately $125-175 per carat, depending on size. Heated bort black diamond should cost in the range of $50 to $75 per carat.

For a brief history of black diamond treatments, including details of the new heat treatment, see "Heat Treated Black Diamond" in GIA' s Gems & Gemology, Fall 2001, p. 214.