corundum, properties.3, gems

An unusual natural cat’s-eye sapphire examined by the GIA Gem Trade Laboratory demonstrates chatoyancy produced by aligned fluid inclusions rather than the rutile needles commonly responsible for asterism in corundum. The specimen, submitted to the West Coast laboratory for identification in early 1995, was a semitransparent oval cabochon weighing 1.91 ct and measuring 13.99 × 5.19 × 2.54 mm. It had a mottled greenish blue colour and displayed a distinct cat’s-eye effect across its domed surface. Standard gemmological testing established its identity as natural sapphire. The note places the observation in the context of the relative frequency of optical phenomena in corundum: asterism is fairly common, whereas chatoyancy is encountered only rarely. In the laboratory’s experience, an apparent cat’s-eye effect in either natural or synthetic corundum usually results from the orientation chosen during cutting of an asteriated stone. When the star’s axis is positioned off-centre, only one ray extends across the cabochon dome, giving the appearance of a single eye. Microscopic examination of the submitted sapphire revealed a different explanation. The stone contained almost no needle-like rutile crystals, prompting investigation of other internal features capable of producing the observed effect. Instead, it contained numerous approximately planar, mutually parallel, liquid-filled inclusions described as fingerprints. The accompanying photomicrograph illustrates these as parallel liquid-filled channels. Light scattering from this aligned inclusion system was identified as the cause of chatoyancy. Examination also revealed diffuse colour halos surrounding crystal inclusions, which were interpreted as evidence that the sapphire had undergone heat treatment. The report therefore documents both an uncommon inclusion-related optical effect and microscopic evidence of treatment in the same natural stone. Its principal finding is that a distinct eye in sapphire can originate from parallel fluid-filled structures, providing an alternative to the more usual explanation involving the cutting orientation of a star-bearing corundum cabochon.