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GIA exposes rough diamond’s fake green color

Manufacturers coated a rough diamond with a green substance to imitate a natural color, the Gemological Institute of America (GIA) said after the stone turned up for grading at its Carlsbad laboratory.

The presence of chromium oxide powder on the surface of the 6.49-carat crystal was a rare case, as color coatings are more common with faceted stones than with rough, and tend to be pink, orange, red or blue, the GIA explained. The recent episode “represents a significant attempt to artificially reproduce the features seen on natural green diamonds,” Virginia Schwartz, a supervisor of diamond identification, and Christopher Breeding, a senior research scientist, wrote in a lab note in the Summer 2019 issue of Gems & Gemology, the institute’s quarterly journal.

Most green diamonds receive their color from radiation damage occurring in the earth’s crust over long periods of time, Schwartz and Breeding continued. Additional radiation can leave green surface patches, which turn brown after exposure to high temperatures. The presence of these “stains” helps indicate the hue is authentic.

However, the stone recently submitted for a Colored Diamond Grading Report gave results during spectroscopic examination that did not match the norm for a natural green diamond. The uneven color came not from radiation marks, but from tiny emerald-green platy crystals attached to the surface. They were removable with tweezers or a pointer probe, but not when wiped with a cloth.

Further testing confirmed the identity of the powder, which workers had heated to about 700 degrees Celsius to produce crystallized plates that resembled genuine radiation stains, the GIA theorized.

“While the unique coating is easily discerned from natural green radiation stains under magnification, this stone is a strong reminder to carefully examine any green diamond, even rough crystals, in order to know exactly what you are buying,” Schwartz and Breeding concluded.

Courtesy - Rapaport

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