In addition to possessing wide variation of color, quartz, like sapphire and certain other gemstones, can exhibit asterism or chatoyancy. The well-known _tiger’s-eye_ from West Griqualand, South Africa, owes its eye effect to the fact that its material is a replacement of fibrous asbestos by cryptocrystalline quartz. The color of tiger’s-eye arises from the partial alteration of the asbestos to yellow-brown iron oxides before it is replaced by quartz. Inclusions of rutile, tourmaline, or actinolite needles may produce attractive patterns in quartz, but they do not always cause chatoyancy. The material containing such inclusions is called sagenitic quartz, or it may be descriptively named, such as rutilated quartz, tourmalinated quartz, and so forth. Sagenitic quartz is usually cut as cabochons rather than as faceted stones since the inclusions are of greater interest than the quartz itself. If the foreign inclusions consist of tiny flakes of hematite or mica, the quartz assumes a spangled appearance and is called _aventurine_. Crystals of quartz varieties that are opaque or that contain visible inclusions normally are cut as cabochons to take advantage of the body color or to make the inclusions more visible. Crystals of the transparent varieties are fashioned in any of several cutting styles, depending on whether it is desired to take maximum advantage of color or of brilliance. Because of its availability in fairly large, flawless pieces in various colors, quartz has been used extensively in carving. The Chinese have excelled in carving large, ornate objects of rock crystal. Although quartz occurs in many varieties and its crystals are cut in many styles, it is easily identified by its refractive index of 1.55, specific gravity of 2.65, and hardness of 7. CRYSTALLINE VARIETIES Amethyst: Purple to violet Cairngorm: Smoky yellow Citrine: Yellow to red-orange and red-brown Milky quartz: White Morion: Black Rock crystal: Colorless Rose quartz: Rose to pink Smoky quartz: Gray to black CRYPTOCRYSTALLINE VARIETIES (CHALCEDONY) Agate: Pronounced color banding Aventurine: Inclusions of sparkling flakes Bloodstone: Dark green dotted with red Carnelian: Red to yellow-red Cat’s-eye: Chatoyant Chrysoprase: Green Jasper: Opaque brown to red-brown, green, yellow, etc. Onyx: Color banding in straight layers of contrasting color Sard: Light to dark brown Sardonyx: Sard or carnelian bands alternating with white bands Tiger’s-eye: Bright brownish yellow, sometimes blue: chatoyant CHRYSOBERYL (INCLUDES ALEXANDRITE AND CAT’S-EYE)