Granite, a coarse-grained rock widely distributed in the earth's crust. Granite is very hard and takes a beautiful polish. The best grades are used for tombstones, statues, and building material.
Several of the Egyptian pyramids are faced with granite from the Nile city of Selene, where quarries existed as early as 3000 B.C. The Romans quarried granite in Wales in the first century A.D. Aberdeen, Scotland, has so many granite buildings that it is called the granite city. Many public buildings in the United States—notably the capitols of Colorado, Pennsylvania, Wisconsin, and Texas—are made of granite.
New England is noted for its granite quarries. (New Hampshire is nicknamed The Granite State.) A light-gray, fine-grained granite from Barre, Vermont, is used throughout the country for gravestones. Granite blocks are also quarried in about 15 states outside of New England. Georgia is the largest producer by volume.
Granite consists mainly of silicic minerals (crystalline compounds of the element silicon), chiefly quartz and feldspar. Mica or hornblende, or both, are also usually present. Granite sometimes contains traces of iron and other minerals. The color of granite varies with its mineral composition; it is usually gray or reddish.
The crystals in granite account for its grainy appearance. (Granite comes from the Latin word granus, meaning grain.). The crystals vary in size, but are usually less than one inch (2.5 cm) long. Pegmatite granite is composed of crystals that may be more than 12 inches (30 cm) long. Granite porphyry has large crystals of quartz or feldspar embedded in a mass of much smaller crystals.
Granite weighs about 12 per cent more than an equal volume of concrete. Granite can withstand pressures of 15,000 to 25,000 pounds per square inch (103,000 to 172,000 kPa), compared to about 4,000 pounds (27,600 kPa) for the strongest concrete.
Granite is usually classed as an igneous rock (one formed by the cooling of molten rock beneath the earth's surface). Some geologists believe that granite comes from sedimentary and metamorphic rocks by a heating and ionizing process called granitization. Granite lies beneath the continents (but not the oceans), and is the core rock of mountain ranges. Huge masses of it have been exposed on the continents by erosion.
Granite is prized as a stone for carving. One of the largest sculptures in the world, Mount Rushmore National Memorial in South Dakota, is carved on a mountainside of granite. The chief use of quarried granite in the United States is for tombstones and mausoleums.
Before the development of reinforced concrete, framing steel, and concrete blocks, most large buildings were set on granite foundations and often had granite load-bearing walls. Architects today use granite mainly in the form of polished slabs to beautify exteriors and interiors.
Granite of inferior quality, and the waste granite in quarries, is crushed into gravel for use on roads and railways.
Granite is expensive to use. Its hardness makes it difficult to quarry, cut to size, and polish. Because of its weight, it is costly to transport.