Just what Wikipedia has to say. 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's eye quartz, or from fibrous inclusions or cavities within the stone, as in cat's eye chrysoberyl. Chatoyancy in wood occurs in various species – particularly hardwoods, and particularly where stresses from the weight of the growing tree result in denser patches, or where stresses cause burl or bird’s eye. This ‘figure’, which has a striking three-dimensional appearance, is highly prized by woodworkers and their clients alike, and is featured regularly in furniture, musical instruments, and other decorative wood products. Figuring takes on a variety of forms and is referred to as flame, ribbon, tiger stripe, quilting, among other names. This effect is sometimes called wet look, since wetting wood with water often displays the chatoyancy, albeit only until the wood dries. Certain finishes cause the chatoyancy (also referred to as wood iridescence, moire, vibrancy, shimmer or glow) to become more pronounced. Oil finishes, epoxy, and shellac can strongly bring out the wet look effect.