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Lab-greening of diamonds by irradiation was first performed by Sir William Crookes in 1904 when he packed conventional diamonds in radium bromide salts for a year or so. These experimental stones remain radioactive to this day and thus cannot be worn. It took until 1942 to green diamonds safely, using a cyclotron to accelerate high-energy particles. By the 1960s, diamond greening was done with streams of electrons in linear accelerators and neutron bombardment in a nuclear reactor.

Irradiation (coupled with heating) can produce colors such as yellow and brown that often look identical to hues produced by nature. Fortunately, these treated colors betray their lab origin by showing absorption lines that are uncharacteristic of stones with natural colors. The reason for the different spectroscope readings between other-than-green natural and treated diamonds is their different color genesis. Excepting green, natural color stems from entrapment of impurity atoms (usually nitrogen) or crystal lattice damage during the creation process while treated color results from irradiation and subsequent heating after a diamond is formed.

On the other hand, nearly all green diamonds—natural or treated—own their color to irradiation (and, to a lesser extent, fluorescence). So whether stones are colored in the earth or in, say, a gamma cell, they will often show the same wavelength absorption lines. This doesn’t mean that experts can’t ever tell apart lab- from ground-greened diamonds. But even when they can, the job is never easy.

Gemologist Stephen Hofer envisions development of simple, effective color origin tests using radioactivity dating techniques to determine when stones were irradiated.”

The Dream Goes On

Given the difficulty of authenticating a green diamond’s color as natural, many dealers long for times when, based on a few signs or even gut instinct, a green diamond could be presumed natural. Such a longing can lead to trouble.

In 1990, a limited number of rough and cut diamonds with pale green colors began to surface in the United Sates. Colored diamond dealers jumped at the opportunity to buy these light and lively stones with hues vastly different from the comparatively somber shades of conventional irradiated diamonds. “They were being offered as natural by people with impeccable credentials,” says Alan Bronstein, Aurora Gems Inc., New York.

From the first, seasoned gemologists were skeptical about these pistachio-green diamonds. “While the color of these stones seems natural, their physical characteristics are not consistent with what researchers think of as natural green color,” Hofer explains. Particularly disturbing: brown irradiation stains on naturals that seemed too weak in comparison to the uniform pale-green body color of the stones.