Topaz, a mineral used both as a gemstone and for the manufacture of mullite, an important heat-resistant material used in dishware, as insulation for spark plugs, and for lining furnaces and boilers. Topaz is an aluminum fluosilicate, consisting of aluminum, silicon, oxygen, hydrogen, and fluorine. It is harder than quartz and usually forms crystals that can be easily shattered when struck. Topaz may be colorless, white, gray, pink, straw-yellow to golden-brown, pale blue, or pale green. It has a glasslike luster, and is transparent or translucent.

Although the best-known shade of gem topaz is wine-yellow (a shade resembling that of sherry wine), precious topaz includes most transparent varieties, regardless of their color. Certain yellow and golden-brown varieties turn blue when exposed to sunlight. Some yellow and yellow-brown varieties turn pink when heated, producing the burnt topaz or Brazilian ruby commonly found in topaz jewelry. When highly polished, topaz has a slippery feeling and, like amber, acquires an electric charge that enables it to pick up bits of paper when rubbed. The wine-yellow topaz is the November birthstone, and represents loyalty. Common topaz is translucent, and usually gray or white. It is not used in jewelry, but only in the manufacture of mullite.

The term topaz is often used to denote less valuable gems that resemble yellow topaz. Spanish topaz, Brazilian topaz, and topaz quartz are names for citrine, a variety of quartz. Oriental topaz is a yellow variety of corundum.

Topaz is usually found near tin-bearing ores, in cavities that occur in rhyolite lavas and granite, and as pebbles in sands of streams. Most wine-yellow crystals are found in Brazil. Beautiful blue, pink, and brown crystals are found in Russia. Many varieties of topaz are found in the United States. Other deposits occur in Japan, Mexico, Burma, Sri Lanka, Nigeria, and Madagascar.

Chemical formula: Al2SiO4(F,OH)2. Specific gravity: 3.4 to 3.6. Hardness: 8.