Quartz is the dominant mineral in sandstone, but other rock grains and mineral grains (especially chalcedony, feldspar, muscovite, hornblende, magnetite, or garnet) generally are present. Sandstones are commonly cemented by carbonates, silica, iron oxides, or clays. Most sandstones are a shade of gray or brown, but the color may vary from gray or white to yellow, brown, or red. The color probably depends on the type of cement, the amount of organic material present, and the amount and degree of oxidation of iron in the rock. Durability of sandstones depends largely on the character of the cement. Some sandstones crumble easily, but others, especially those cemented by iron oxides or silica, are tough and durable. Sandstones break around the grains, giving the broken surface a granular appearance. Sandstone crops out in many places throughout much of the state. In LaSalle and Ogle Counties, large tonnages of sand are mined from the St. Peter Sandstone and sold for a variety of uses, including abrasive sand, molding sand, and sand for making glass. In extreme southern Illinois attractively colored sandstones have been quarried for building stone. QUARTZITE (9) QUARTZITE is a metamorphic rock that originally was quartz sandstone. Quartzites are produced by intense heat and/or pressure, probably aided by hot silica-bearing solutions. The quartz grains may be so closely interlocked that individual grains are no longer recognizable. The rock fractures conchoidally through both the grains and cement, so the broken surface, unlike that of sandstone, is smooth and may even be glassy like quartz. Color depends upon the amount and kind of impurities present. A quartzite that is all quartz is white or gray, but iron or other elements may change the color to shades of purple, yellow, brown, or red. Quartzite is a very resistant, hard rock and cannot be scratched by a knife. Quartzite is abundant as boulders and pebbles in glacial drift of Illinois, having been brought into the state during the “Ice Age.” SHALE (10) SHALE is a common and important sedimentary rock composed of compacted clay or mud. It is so fine grained that the minerals forming it generally cannot be identified without the aid of X-ray. Shales are composed mainly of clay minerals but, like other sedimentary rocks, generally include other minerals. Shales containing calcium carbonate are called calcareous shales. Most shales contain silt or sand particles; if silt or sand is present in large quantity, the rock is called silty shale or sandy shale. If mica minerals are present in quantity in a shale it is called micaceous shale. The particles of most clay minerals are thin and flat and overlap each other.