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In gemology, chatoyancy is an optical reflectance effect seen in certain gemstones. Coined from the French, meaning "cat's eye," chatoyancy arises either from the fibrous structure of a material, as in tiger eye quartz, or from fibrous inclusions or cavities within the stone, as in cat's eye chrysoberyl. The effect can be likened to the sheen off a spool of silk; the mobile, wavering reflection always being perpendicular to the direction of the fibres. For a gemstone to show this effect it must be cut en cabochon, with the fibres or fibrous structures parallel to the base of the finished stone.

Some gem species known for this phenomenon include the aforementioned quartz, chrysoberyl, beryl (especially var. aquamarine), tourmaline, apatite, and scapolite.

See also: asterism, optical phenomenon