Cat's Eye

Not a single mineral species, but rather an optical phenomenon ("chatoyancy") caused by parallel fibres included within another mineral species, most commonly quartz. Also sometimes seen in tourmaline, apatite, corundum, chrysoberyl, beryl (especially emerald), danburite and other minerals. To avoid confusion, the name "cat's eye" should always be used as a modifier with a mineral species or gem name, not by itself.