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The East Coast lab recently received for identification two asteriated dark blue oval cabochons that weighed 31.64 ct and 15.26 ct, and measured 19.70 × 16.31 × 8.64 mm and 14.88 × 12.49 × 6.68 mm, respectively. In overhead reflected light, both cabochons appeared to be evenly colored and nearly opaque. The stars appeared white, with straight arms of equal length that seemed to float over the surface of the stones. Typically, a six-rayed star forms as the result of reflections off three sets of oriented needles that are present throughout a large portion of the stone. Although examination in transmitted light revealed opaque and translucent areas along with straight, angular color banding, we did not see any oriented needle-like inclusions with magnification. Aware that diffusion-induced asterism has been around since Linde Air Products Company patented the process in 1954 (as described in K. Nassau, “Heat treating ruby and sapphire: Technical aspects,” Fall 1981 Gems & Gemology, pp. 121–131), we suspected that the two stones had been treated.
Standard gemological testing produced the following information on both stones: spot refractive indices of 1.76, no fluorescence to either long- or short-wave UV radiation, and a solid, heavy band from 450 to 470 nm that was visible in the desk-model spectroscope and was probably due to trace amounts of iron. Standard magnification (10×–60×) revealed melted fingerprint-like inclusions, polysynthetic twinning nearly parallel to the bases of the cabochons, and (as noted above) straight, angular bands that were either a very saturated dark blue or colorless—but no oriented needle-like inclusions. This combination of properties and features indicated that the stones were natural sapphires but the stars were manufactured.
In a sapphire with a natural star, the orientation of the asterism-causing needles is directly related to the crystallography of the host stone. Because color zoning is also related to the hexagonal crystal structure, the rays of the stars are always parallel to the six sides of the hexagon created by the growth/color zoning. Even if only one section of zoning is visible, as seen in the smaller of the two cabochons, two of the star’s six rays would still be parallel to the visible banding. As is evident in the figure, however, that was not the case for the stars on these cabochons. Therefore, we were not surprised when fiber-optic lighting revealed a very shallow hazy, whitish surface layer on both stones. Such a layer, composed of tiny individual needles too small to be detected at standard gemological magnification, previously has been associated with diffusion-induced stars (see, e.g., Lab Notes—Summer 1982, pp.106–107; Summer 1985, pp. 112–113; Fall 1985, pp. 171–172; Spring 1991, pp. 44–45; and Spring 1995, pp. 56–57; and Gem News—Summer 1996, pp. 136–137; and Winter 1997, pp. 308–309). Combined, the above findings allowed us to conclude that the stars on these cabochons had been artificially applied by means of a surface diffusion process.
It is particularly interesting that the treatment in these two cabochons consisted only of the creation of the stars, since the Gem Trade Laboratory more commonly sees this process on cabochons in which the color also has been diffusion-induced.
Wendi Mayerson
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