September 1999

Diamonds/Gemology

Hot Stuff

This rarity among diamonds blushes in the heat

The magic of a living chameleon lies in its ability to change color to hide from predators. In the gem world, a chameleon diamond has no predators – but many admirers. The magic is that it changes color dramatically when heated and slowly reverts to theoriginal color as it returns to room temperature.

Collector's Item
This kind of diamond – hydrogen-rich Type Ia (for a discussion of diamond types, see Professional Jeweler, August 1999, p. 30) – is rare and desired mainly by collectors.

The charming characteristic would be lost in mounted jewelry, and it would be a bad idea to approach the wearer with a Bunsen burner, asking to witness her diamond changing hues. For the jeweler, however, knowing about chameleons and wowing your customers with amazing data about them can add to the romance of diamonds.

"Chameleons can go from their stable, generally grayish-green color to a bright yellow," writes Dr. Emmanuel Fritsch in The Nature of Diamonds (Cambridge University Press, Cambridge, England, 1997). "Chameleon diamonds have unusually high absorption bands at the red end of the visible spectrum (550 nanometers), causing the green stable color. This green color is caused by hydrogen-containing defects."

By Degrees
Heating these diamonds at 200°-300°C (392°-572°F) with an alcohol lamp causes changes in the absorption band. "Heating the diamond – or keeping it in the dark for an extended period – reduces the absorption, leaving the more saturated underlying yellow visible," he says. The yellow is caused by nitrogen-related color centers, he adds.

– by Robert Weldon,G.G.