Extensive deposits of fluorite, one of Illinois’ important commercial minerals, occur in Hardin and Pope Counties in extreme southern Illinois, where it is associated with galena, sphalerite, calcite, barite, and other less abundant minerals. Fluorite is used to make hydrofluoric acid, to form a fluid slag in the production of iron and steel, in the manufacture of aluminum, to make many chemical products, and in the ceramic industry, in which it is used to make colored glass, enamels, and glazes. GYPSUM (26) GYPSUM, hydrous calcium sulfate, is a colorless, transparent to translucent mineral when pure, but it often is stained yellow by impurities. It has a white streak, is soft enough to be scratched by a fingernail, and is light weight. [Illustration: uncaptioned] Gypsum occurs in several forms. Selenite is a coarsely crystalline, transparent variety, composed of flat, nearly diamond-shaped crystals that can be split easily into thin sheets, have a glassy luster, and often grow together to form “fishtail twins.” Crystals of selenite occur in shales of the “Coal Measures” of southern, north-central, and western Illinois, and can be picked up at the surface. Satin spar has crystals like silky threads closely packed together, splits parallel to the fibers, and is found as fillings in rock cracks and as thin layers in shales. Massive gypsum is granular. Gypsum deposits occur deep underground in Illinois but thus far have not been mined. CHERT (27) CHERT, one of the main forms of silicon dioxide, is cryptocrystalline (submicroscopic) quartz. Most of the chert in Illinois is white or gray, but impurities stain many deposits yellow, brown, or even pink. Chert is so hard that it can scratch glass and ordinary steel. It is fairly light weight, dense, opaque, and brittle; the luster is dull. Flint, a variety of chert, is generally dark colored, more dense, may have a glassy luster and be transparent in thin flakes. Both chert and flint have a smooth, curved (conchoidal) fracture, but flint tends to break with thinner, sharper edges. Indians used flint and chert to make arrow points and spearheads. Chert occurs as rounded masses (nodules and concretions) or as irregular layers in limestones and dolomites in northern, western, southwestern, and southern Illinois. Because chert is hard and more resistant to weathering than limestone or dolomite, it often remains after the rest of the rock has weathered away. Chert also is abundant in many glacial deposits because it is hard and resists solution. Streams that flow through cherty bedrock or glacial deposits carry pebbles along and concentrate them as gravel in stream channels. Cherty stream gravels are especially abundant in western and southern Illinois.