In gemology, Chatoyancy is the term used when a gemstone shows the cat’s eye effect. Chatoyancy is caused by fine fiber-like inclusions in a gemstone which channel the reflecting light into a narrow band that resembles the eye of a cat. Of all the gemstones that exhibit cat’s eye, Chrysoberyl Cat’s Eye is the best known. Chrysoberyl, of which Alexandrite is a member, has the strongest and most distinct cat’s eye effect of all gemstones. Other chatoyant gemstones are Quartz (tiger’s eye), Beryl (Aquamarine and Emerald), Tourmaline and Moonstone. Cat’s Eye are usually cut in a smooth cabochon cut to enhance the phenomenon. The name “Cat’s Eye” by itself refers only to the Chrysoberyl cat’s eye. Any other gemstone exhibiting the cat’s eye effect will also have the name of the gemstone, for example, “Tourmaline cat’s eye.”
Similar to chatoyancy is Asterism, where instead of a single band channeling the light there is a brighter star shape in the stone. Sapphires exhibiting asterism are generally the most popular but many other gemstones can be found with this phenomenon. Some of the known star gemstones include: corundum (ruby, sapphire), quartz, garnet, spinel, chrysoberyl, and moonstone.