In gemology, chatoyancy [shuh-toi-an-see], or chatoyance, 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.
The Daily Omnivore 