Combination of Phenomena in Star and Cat’s-Eye Color-Change Sapphire
The Tokyo lab recently examined a transparent 20.49 ct violet to purple color-change double cabochon sapphire, measuring 16.67 × 14.69 × 8.51 mm (figure 1) with a specific gravity of 3.99 and a spot refractive index of 1.77. Usually a star sapphire is only polished on one side, and the unpolished side does not show any phenomena. But this stone was polished on both sides, which resulted in a unique combination of phenomena. Side A displayed a six-rayed star. Side B also displayed a six-rayed star, but one line containing two of the rays was much stronger, which made the phenomenon reminiscent of a cat’s-eye.
Chatoyancy is displayed when light reflects off a set of dense parallel elongated inclusions, such as needles; asterism occurs when such a set of inclusions is oriented in multiple directions. In the case of this stone, side A with the distinct six-rayed star had three sets of dense parallel needles and platelets close to the surface, intersecting at 60° angles (figure 2), which is common with six-rayed asterism. Side B also had three sets of these needles close to the surface, but the needles were more concentrated in one of the directions, producing a cat’s-eye effect (figures 2 and 3). Interestingly, the chatoyancy in Side B and one line of asterism in Side A are contributed by the same group of needles.
Although color-change star sapphire is not particularly rare, color-change cat’s-eye sapphire is far less common. Color-change sapphire with asterism on one side and chatoyancy on the other is without question a “phenomenal” gem.
