Sand is a granular material composed of finely divided rock and mineral particles. Sand has different compositions, but is defined by its grain size. Grains of sand are smaller than gravel and coarser than silt. Sand can also refer to a soil texture class or soil type; that is, soil that contains more than 85 percent by mass of sand-sized particles. Sand composition varies depending on local rock sources and conditions, but the most common component of sand in continental and non-tropical inland coastal areas is silicon dioxide (silica or SiO2), usually in the form of quartz. Calcium carbonate is the second most common type of sand, for example aragonite, created mainly by various life forms, such as corals and crustaceans, over the last 500 million years. For example, it is the major form of sand found in areas where coral reefs have dominated the ecosystem for millions of years, such as B. in the Caribbean. More rarely, the sand may be composed of calcium sulfate such as gypsum and selenite, as found in places like White Sands National Park and the Salt Plains National Wildlife Refuge in the United States. Sand is a non-renewable resource on a human scale and sand suitable for concrete production is in great demand. Desert sand, while plentiful, is not suitable for concrete. Every year 50 billion tons of beach sand and fossil sand are used for construction.