In spring 1997, the West Coast lab received for identification the brownish yellow cat's-eye shown in figure 6. The chatoyancy in this 9.00 ct oval double cabochon resulted from numerous parallel 'needles'--both fine and coarse--which were oriented across the width of the stone. A spot R.I. of 1.45 and a specific gravity of 2.08, obtained hydrostatically, pointed to opal as the bulk material, notwithstanding that brown is an unusual color for opal. Opal is singly refractive, but phenomenal opal often exhibits anomalous double refraction between crossed polarizers. In this case, observation down the length of the cabochon revealed an indistinct uniaxial optic figure. The gem also displayed distinct dichroism, with one color more yellow and the other more brown. The cabochon was inert to long-wave ultraviolet radiation and fluoresced weak red to short-wave UV. It had a slightly resinous luster.

This combination of properties closely matches those of a chatoyant opal described in the Fall 1990 Gem News (pp. 232-233). In that report, the authors suggested that the opal could be a pseudomorph after goethite, and that the inclusions were most likely remnants of goethite. GIA Research analyzed the present cabochon using EDXRF, and found silicon, iron, potassium, calcium, and zinc. The presence of iron supports the idea that the included needles could be goethite.

The brownish yellow color of this cabochon (which appears to be directly related to the inclusions) is similar to the dark brown color described for the treated-color cat's-eye chrysoberyls that made news last fall because of their residual radioactivity. However, the properties of opal (luster, R.I., and specific gravity) are very different from those of chrysoberyl. Cat's-eye opal is not known to be irradiated, or to show any residual radioactivity.

Photo GIA