Lab Notes Gems & Gemology, Fall 2020, Vol. 56, No. 3

Color-Change Cat’s-Eye Diaspore

Taku Okada

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Color-change cat’s-eye diaspore.
 
 
 
 
 
 
Figure 1. The 60.71 ct diaspore displaying color change and chatoyancy under fluorescent light (left) and incandescent light (right). Photos by Shunsuke Nagai.

A 60.71 ct cabochon was recently submitted to the Tokyo lab for identification (figure 1). Diaspore, α-AlO(OH), is one of the main aluminum minerals in bauxites, along with gibbsite, γAl(OH)3, and boehmite, γAlO(OH). It is often observed as inclusions in rubies. Diaspore is also a gem in its own right, although it is very brittle and has perfect cleavage. The birefringence blink method revealed refractive indices of 1.69 and 1.74 and that the stone was biaxial. A dichroscope revealed strong trichroism: yellowish green, purple, and brown. The Raman spectrum was consistent with diaspore in the RRUFF reference database (R060287).

The hue of this stone changed from greenish yellow under daylight or fluorescent light to pinkish brown under incandescent light. Diaspore’s color change can be caused by the chromophores Cr3+, V3+, and Fe2+–Ti4+ pairs (C. Shen and R. Lu, “The color origin of gem diaspore: Correlation to corundum,” Winter 2018 G&G, pp. 394–403). Color-change diaspores are sometimes known in the trade as “Zultanite” or “Csarite.”

Needles and thin films produce the cat’s-eye effect in this diaspore.
 
 
 
 
 
 
Figure 2. Needles and/or tubes with iridescent reflective thin films or fissures producing chatoyancy under brightfield and overhead illumination (left) and overhead illumination only (right). Photomicrographs by Taku Okada; field of view 13.60 mm.

Chatoyant phenomena are usually the result of many long, thin, and closely spaced parallel needles or fibers, combined with cutting of a cabochon in the correct orientation to optimize the reflection of light off of these parallel inclusions. In this stone, the chatoyancy was caused not only by needles and/or tubes but also by the combination of iridescent reflective thin films or fissures in various shapes from ultrafine to wide, aligned along repeated cleavages (figure 2). Although some fissures reached the surface, no clarity enhancement was detected in this diaspore.

Taku Okada is staff gemologist at GIA in Tokyo.