This 6.49-carat rough “green” diamond was submitted to GIA’s Carlsbad laboratory for a Colored Diamond Grading report but turned out to be coated with emerald-green “platy crystals” and not naturally irradiated, like most green diamonds. (Photo credit: Diego Sanchez)
Carlsbad, Calif.—The Gemological Institute of America’s Carlsbad laboratory recently spotted a natural rough diamond covered in a layer of emerald-green crystals intended to imitate the color of a green diamond.

A client submitted a 6.49-carat green crystal for a Colored Diamond Grading Report, and the surface of the stone was covered by uneven patches of green color, according to a lab note written by Virginia Schwartz and Christopher Breeding that appeared in the Summer 2019 issue of Gems & Gemology.

An analysis by the lab revealed a Type Ia diamond, but GIA noted that the ultraviolet-visible spectrum was “very unusual” and didn’t show any bands produced by radiation damage.

Most natural green diamonds are colored by radiation damage, which often brings with it “surface patches” of green or brown color that gemologists refer to as “radiation stains.”

These stains are important, Schwartz and Breeding pointed out, because they indicate a diamond has been exposed to a source of natural radiation.

After examining the diamond, they discovered that its green color was not, in fact, due to radiation stains but to groupings of emerald-green “platy crystals”—identified through Raman analysis as chromium oxide—attached to the diamond’s surface.

The individual crystals were easily removed with tweezers or a pointer probe but stayed attached to the surface when they underwent normal actions, like wiping the diamond with a cloth.

The researchers said most colored coatings they see in the lab are on faceted stones and are pink, orange, red or blue.

They added that the use of chromium oxide powder to create a green coating represents “a significant attempt to artificially reproduce the features seen on natural green diamonds.”

GIA researchers also noted that even though the coating can easily be distinguished from natural green radiation stains under magnification, the diamond is “a strong reminder to carefully examine any green diamond, even rough crystals, in order to know exactly what you are buying.”

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