Sand, small particles of rock and minerals or other materials, such as coral or slag. The term sand refers to the size of the particles, not to their composition. There are, however, no universally accepted limits for the size of sand particles. Geologists define sand as those particles between .0025 inch and .079 inch (.064 mm and 2 mm) in diameter. Sand for industrial use is defined as particles between .0029 inch and .25 inch (.074 mm and 6.35 mm) in diameter. In everyday usage, sand is considered to consist of particles just large enough to be seen individually with the unaided eye.
Sand is used for many purposes. Quartz sand is melted down to make glass. Various types of sand are used in cement and concrete for building materials and paving. Sand is a natural abrasive and is used for cleaning, etching, stonecutting, and smoothing in sandblasting and is also used in making sandpaper. Water and other liquids are filtered through sand to remove suspended matter. In gardening, sand is sometimes added to clay soils to improve their aeration and drainage.
Sand is found over much of the earth—in soils, on ocean floors, on beaches, in deserts, and along streambeds. Most sand consists of quartz (silicon dioxide). Few deposits, however, contain pure quartz sand. Usually other minerals, such as feldspar, mica, magnetite, and garnet, are mixed with the quartz; they account for the particular color of any given sample of sand. There are some places where the sand contains no quartz. The white sand of White Sands National Monument, in New Mexico, is pure gypsum. Some beaches in Florida and on some of the Caribbean islands are made up of coral sands.
Sand can be manufactured by pulverizing any sufficiently hard material such as rock or slag. Naturally occurring sand is the product of wind and other agents of erosion. Weathering breaks down the parent material into particles of various sizes, including sand. Wind and water carry the particles to new locations, where deposits build up.
The texture of the sand in a particular deposit depends on the way in which the sand particles were transported. The ability of water to carry suspended particles, such as sand, depends on its rate of flow. A swiftly moving stream, for example, may pick up particles of assorted sizes, carry them downstream, and then drop them at a point where a bend in the stream or an obstruction slows the water. At such points, deposits of mixed texture tend to build up.
Wind of moderate velocity sorts out the particles by size. Particles larger than sand are generally not moved, while dust particles (smaller than sand) are lifted high in the air and carried for great distances. Sand particles, on the other hand, skip along the ground and build up in front of almost any obstacle. In areas of prevailing winds sand builds up into dunes, such as those at Great Sand Dunes National Park, in Colorado.
Sand deposits may be laid down, compressed, and cemented together to form rock, usually sandstone. If it does not become rock, sand may be continually moved about, worn down, and finally become so small that it ceases to be sand and becomes dust.