No fancy color diamond collection is complete without a green stone. But since, next to red, green is probably the rarest of all natural diamond hues, most collections lack representation from the green portion of the diamond rainbow. Even when they do contain greens, the stones’ color is usually not a bona fide natural one—or at least not classified as such by a gem lab of stature.
Needless to say, this is a rather frustrating situation for connoisseurs. And it arises out of the highly ironic fact that green, while among the rarest of natural diamond colors, is the commonest of artificial ones—easily induced by alpha, electron, gamma and neutron irradiation. Yet connoisseurs spurn stones with lab-contrived color, instead dreaming of some day owning a green diamond with incontestably natural color.
As things stand, that’s dreaming a nearly impossible dream. Since 1985, only a very few diamonds with color certified as being of natural origin have been sold at auction. Two of the most notable: a 51-point dark green sold for $69,000 per carat at Christie’s in December 1985 and a 3.02-carat “apple-green” sold for $568,000 per carat at Sotheby’s in April 1988.
In the past, labs were loath to validate green in diamonds as natural unless stones showed what was once thought to be a telltale indicator of natural color origin: tiny circular green and/or brown stains seen with magnification on the unpolished surfaces, called naturals, of finished stones. For instance, the 3.02-carat stone just referred to had such stains (in a girdle fracture) and was accompanied by an April 1985 GIA report with the following comment: “Color and characteristics of this stone suggest natural color.”
Today, however, the same evidence of natural color origin would probably not be enough to merit a green diamond a comment this affirmative from GIA. Diamond graders there say that about the best such a stone can hope for at present from the school’s trade labs is to have its color origin classified as “undetermined.” While such wording may smack of fence-sitting, it is understandable since GIA feels that definitive proof—not merely educated opinion—is expected of it when making complicated color origin calls. Definite proof may be years away. In its absence, colored diamond experts think that both the gemological and connoisseur worlds must learn to live with educated opinion.
It Isn't Easy Being Green
Unlike other natural colored diamonds, the cause of color in green stones is thought to result from natural irradiation in the earth—most likely after the stones’ formation. According to current diamond color theory, some time in a diamond’s history it comes into contact with a mineral (e.g., pitchblende) containing radioactive elements such as uranium whose high-energy particles create defects in the stone’s atomic structure.
These radiation-induced defects are, in turn, responsible for the absorption of wavelengths of red light by the diamond that result in the transmission of a complementary green color to the eye. During an examination of Germany’s famous 41-carat-plus Dresden diamond—to natural green diamonds what the Hope diamond is to natural blues—a team of three gemologists found that spectroscopic analysis of the diamond confirmed this explanation of green diamond color chemistry.