is a blue-gray to blue-green fibrous variety of opaque quartz formed when quartz replaces the mineral crocidolite. Like tiger's eye, hawk's eye exhibits chatoyant stripes and a silky luster. When cut en cabochon, the surface of hawk's eye shows chatoyancy reminiscent of the eye of a cat. Pietersite, another popular blue-gray gemstone, is made up mainly of hawk's eye and tiger's eye. The most important sources for hawk's eye include South America, India, Namibia, Australia, Burma (Myanmar) and the United States.