Chatoyancy

Chatoyancy occurs when densely packed parallel inclusions, fibers, or hollow tubes reflect light in a single direction. The effect is most striking in chrysoberyl cat's eye, where a single sharp band glides across the stone's surface. To display chatoyancy, the stone must be cut as a cabochon with the inclusion planes perpendicular to the base and the long axis parallel to the inclusions. Less pronounced chatoyancy is also seen in tourmaline, aquamarine, and some quartz varieties. In the trade, the unqualified term 'cat's eye' refers exclusively to chrysoberyl cat's eye.