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Figure 1: This 10.12 ct. sample proved to be a heat-treated Kashan flux-grown synthetic ruby. Photo by S. Singbamroong, © Dubai Gemstone Laboratory.
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In March 2006, a 10.12 ct. transparent purplish red oval mixed cut (figure 1) was submitted for identification to the Dubai Gemstone Laboratory. The client indicated that it had been purchased in and originated from Myanmar.
Standard gemological testing established the following properties: refractive index—1.762–1.770; birefringence—0.008; optic sign—uniaxial negative; pleochroism—moderate orangy red to purplish red; specific gravity (determined hydrostatically)—3.98; fluorescence—strong red to long-wave and moderate red to short-wave UV radiation, with a strong chalky blue luminescence on the surface; and "chrome" lines seen in the absorption spectrum with a desk-model spectroscope. These properties were consistent with ruby, while the strong chalky blue surface fluorescence to short-wave UV radiation suggested heat treatment.
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Figure 2: The synthetic ruby contained internal features that resemble those seen in flux-assisted heated natural rubies. Photomicrograph by S. Singbamroong, © Dubai Gemstone Laboratory; magnified 6x.
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When examined with magnification, this sample at first showed internal features that looked very much like those seen in flux-assisted heated natural rubies (figure 2). However, when examined carefully, these inclusions proved to be various forms of flux residue, such as are found in flux-grown synthetics. Also apparent were white, high-relief, parallel rods; feather-like structures or "fingerprints"; rain-like structures resembling comets; and discoid features with so-called paint splash inclusions. These inclusions are typical of Kashan flux-grown synthetic. Also present were dissolved white flux-filled negative crystals with associated discoid fissures, which are indicative of heat treatment.
UV-Vis absorption spectroscopy showed broad bands centered at 410 and 560 nm, and a small peak at 694 nm; these are typically responsible for ruby color. However, the spectrum also showed UV transmission at 305 nm that was stronger than the visible-region transmission at ~475 nm (blue region), which is suggestive of synthetic ruby. Infrared spectroscopy revealed no peaks related to the OH-group (hydroxyl), as expected for a flux-grown synthetic. EDXRF analysis revealed the low amounts of Fe, a V, and Ga, combined with slightly higher amounts of Ti, that are distinctive of Kashan flux-grown material.
Since the 1990s, a variety of heat-treated synthetic rubies have been reported. Heat treatment of synthetic ruby makes identification more difficult and complicated. Thus, more careful examination and sophisticated testing were necessary to complete this identification.
Sutas Singbamroong and Nazar Ahmed
Dubai Gemstone Laboratory
United Arab Emirates
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