In
gemology,
chatoyancy ( ), 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. The precipitates that cause chatoyance in chrysoberyl are the mineral
rutile, composed mostly of
titanium dioxide. Examined samples have yielded no evidence of tubes or fibres. The rutile precipitates all align 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.