Chatoyancy

In gemology, chatoyancy ( /ʃəˈtɔɪ.ənsi/ 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 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 show this effect best it must be cut en cabochon, with the fibers or fibrous structures parallel to the base of the finished stone. Faceted stones are less likely to show the effect well.