Chatoyancy

In gemology, chatoyancy (/ʃəˈtɔɪ.ənsi/ shə-TOY-ən-see), also called chatoyance or the cat's-eye effect, is an optical phenomenon in which a narrow, luminous band of reflected light appears across the surface of certain gemstones. The band resembles the slit pupil of a cat's eye and appears to glide across the surface as the stone, the light source, or the observer moves. The same terminology is used also in wood science to describe the phenomenon happening in some valuable wood species, like genuine mahoganies, khaya, sapele, sipo etc.

The term derives from the French œil de chat ("cat's eye"). Although historically used primarily in gemology, analogous optical effects occur in other fibrous or anisotropic materials, including wood, where they are commonly described as a special wood figure, or even chatoyance.

Chatoyancy results from the directional reflection of light by parallel fibrous structures or aligned needle-like inclusions within a material. In tiger's eye, the effect is produced by parallel fibrous crystals, whereas in cat's-eye chrysoberyl it is caused by parallel microscopic inclusions or hollow tubes, commonly composed of or associated with rutile (titanium dioxide).