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In gemology, chatoyancy (// shə-TOY-ən-see), or chatoyance or cat's eye effect, is an optical reflectance effect seen in certain gemstones. Coined from the French "œil de chat," meaning "cat's eye," chatoyancy arises either from the fibrous structure of a material, as in tiger's eye quartz, or from fibrous inclusions or cavities within the stone, as in cat's eye chrysoberyl. Marder and Mitchell demonstrated that the precipitates that cause chatoyance in chrysoberyl are the mineral rutile, composed mostly of titanium dioxide. There was no evidence of tubes or fibers in the samples examined. The rutile precipitates were all aligned perpendicularly with respect to cat's eye effect. It is reasoned that the lattice parameter of the rutile matches only one of the three orthorhombic crystal axes of the chrysoberyl, resulting in preferred alignment along that direction.
The effect can be likened to the sheen off a spool of silk: The luminous streak of reflected light is always perpendicular to the direction of the fibres. For a gemstone to show this effect best it must be cut en cabochon, with the fibers or fibrous structures parallel to the base of the finished stone. The best finished specimens show a single sharply defined band of light that moves across the stone when it is rotated. Chatoyant stones of lesser quality display a banded effect as is typical with cat's-eye varieties of quartz. Faceted stones do not show the effect well.
Gem species known for this phenomenon include the aforementioned quartz, chrysoberyl, beryl (especially var. aquamarine), tourmaline, apatite, moonstone and scapolite. Glass optical cable can also display chatoyancy if properly cut, and has become a popular decorative material in a variety of vivid colors.
The term "cat's eye", when used by itself as the name of a gemstone, refers to a cat's eye chrysoberyl. It is also used as an adjective which indicates the chatoyance phenomenon in another stone, e.g., cat's eye aquamarine.
Chatoyancy can also be used to refer to a similar effect in woodworking, where certain finishes will cause the wood grain to achieve a striking three-dimensional appearance; this can also be called pop-the-grain, wood iridescence, moire, vibrancy, shimmer or glow. This effect is often highly sought after, and is sometimes referred to as "wet look", since wetting wood with water often displays the chatoyancy, albeit only until the wood dries. Oil finishes, epoxy, and shellac can strongly bring out the "wet look" effect.