Lab Notes Gems & Gemology, Spring 2015, Vol. 51, No. 1

Artificially Irradiated Color-Change Diamonds

Color-treated chameleon diamonds
Figure 1. These two chameleon diamonds (0.35 and 0.27 ct) were color-treated with artificial irradiation. Photo by Jian Xin (Jae) Liao.

Diamonds that exhibit a temporary color change, commonly referred to as “chameleon” diamonds, are rare in nature. Their color changes with gentle heating, or when they are left in darkness for a period of time. The change from a dark greenish to a lighter yellow hue upon gentle heating is due to the thermochromic properties of these diamonds (D.J. Content, Ed., A Green Diamond: A Study of Chameleonism, W.S. Maney & Son, Leeds, England, 1995, 42 pp.).

Recently the New York laboratory examined two chameleon diamonds, a 0.35 ct Fancy Deep yellow-green marquise and a 0.27 ct Fancy Deep grayish yellowish green marquise (figure 1). Spectroscopic analysis and gemological observations confirmed that these were typical chameleon diamonds. After excitation with short-wave UV light, both exhibited a strong blue to yellow phosphorescence often seen in natural chameleon diamonds. Their UV-Vis absorption spectra showed a broad band at about 480 nm, as expected for this type of diamond. But the spectra also displayed a peak at 741 nm (figure 2), known as GR1 (general radiation damage), that can contribute to a green color in diamonds. Because this radiation-related feature is not found in untreated natural chameleon diamonds, we concluded that both stones had been artificially irradiated.

Chameleon diamonds’ spectra
Figure 2. The UV-Vis spectra for the two chameleon diamonds show a broad absorption peak around 480 nm and an atypical GR1 feature at 741 nm.

A chameleon diamond is an unlikely candidate for artificial irradiation treatment to enhance the stone’s green or blue bodycolor. It is likely that this property was not known or understood before the irradiation process. A permanent color change may occur if these treated chameleon stones are heated for a prolonged period, so it is important to exercise caution during testing. Because the diamonds’ original colors are unknown, they were issued reports stating that they had been artificially irradiated to enhance their color.

Sally Chan and Jessie Yixin Zhou are staff gemologists, and Paul Johnson is a researcher, at GIA’s New York laboratory.