Lab Notes Gems & Gemology, Fall 2016, Vol. 52, No. 3

Very Large Artificially Irradiated Yellow Diamond

59.88 ct Fancy Vivid yellow irradiated diamond
Figure 1. This 59.88 ct Fancy Vivid yellow diamond was treated through artificial irradiation and annealing. Photo by Jian Xin (Jae) Liao.

Artificial irradiation and annealing to enhance or change color is one of the oldest diamond treatments. English chemist Sir William Crookes first discovered the effects of radiation on a diamond’s color, conducting experiments using radium salts in 1904. Today, only a very small percentage of all natural diamonds are irradiated, typically with an electron beam. Because of the risk of damage, diamonds subjected to this treatment are typically under 10 carats.

A 59.88 ct yellow diamond (figure 1) recently examined at the New York laboratory received clarity and color grades of SI1 and Fancy Vivid yellow. While the face-up color distribution was classified as even, we observed a moderate color concentration along the culet typical of some artificially irradiated diamonds (figure 2). Microscopic examination showed noticeable crystal inclusions at 10× magnification as well as extensive burn marks across the entire surface of the stone. The diamond showed chalky blue fluorescence under long-wave UV excitation and medium yellow fluorescence under short-wave UV.

Color concentration along a diamond’s culet, evidence of irradiation
Figure 2. Strong color concentration along the diamond’s culet is evidence of artificial irradiation. Photo by Christopher Vendrell.

Using a desktop spectroscope, the diamond’s visible absorption spectrum revealed a series of absorption lines between 415 and 477 nm, indicative of a “cape” diamond. Additionally, an unusually strong peak at 503.2 nm (H3) was noted using a UV-visible spectrometer (figure 3). The enhancement of this H3 center is responsible for the very desirable yellow color. The infrared absorption spectrum showed high concentrations of nitrogen in the one-phonon region, indicating type Ia diamond. Further examination of the infrared region revealed a minor hydrogen defect as well as weak to moderate peaks at approximately 4935 cm–1 (H1b) and 5165 cm–1 (H1c), further evidence of treatment.

Vis-NIR absorption spectrum of a very large irradiated yellow diamond
Figure 3. The yellow diamond’s Vis-NIR absorption spectrum shows a strong peak corresponding to the H3 defect observed at about 503 nm.

These features led to the conclusion that the diamond was artificially irradiated and annealed (by introducing the H3 defect) to induce a more desirable yellow color. This was by far one of the largest artificially irradiated diamonds identified at GIA, surpassing previous examples by several carats (see Summer 2012 Lab Notes, p. 132; Winter 2014 Lab Notes, p. 295). This diamond reiterates the importance of careful analysis, since even the largest stones may have undergone treatment.

Christopher Vendrell is a staff gemologist, and Paul Johnson is a supervisor of diamond advanced testing, at GIA’s laboratory in New York.