In gemology, chatoyancy (pron.: // 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 rutile,
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