Chatoyancy, or “cat's eye,” is an optical phenomenon caused by light reflecting off a gemstone with internal fibrous minerals or inclusions arranged in parallel direction. When these gemstones are cut in specific directions, the reflected light display a pattern that is perpendicular to the fibrous streaks, which is similar to that a cat's eye. Gemstones displaying chatoyancy are usually cut en cabochon , and their values are determined by color and the qualities of the chatoyancy. A good chatoyancy should be straight, finely sliced, bright, in the center of the jewel, and lively. Gemstones with chatoyancy qualities include chrysoberyl, tourmaline, aquamarine, moonstone, jade, scapolite, quartz, and diopside. Cymophane, a type of translucent yellow chrysoberyl, is the rarest and most valuable chatoyant gemstone. Jadeite produced in Feng Tian of Hualien , Taiwan may also possess some chatoyancy, which is called “Cat's eye nephrite.” Tiger's eye is another type of chatoyant gemstone that owes its light phenomenon to the replacement of fibrous crocidolite with quartz.