Limestone is a very common type of soft, slightly soluble sedimentary rock that is composed principally of calcium carbonate, or the double carbonate of calcium and magnesium. Most of it was formed in shallow marine environments, such as continental shelves when these minerals precipitate out of water containing dissolved calcium. This can take place through both nonbiological and biological processes, especially through the accumulation of corals and shells in shallow seas during the past 540 million years. Limestone often contains tiny fossils, shell fragments and other fossilized debris, which provide information both about the origin of the limestone and about ancient environments and the evolution of life.
Limestone is exposed over large regions of the earth's surface and it plays an important role in soil and other local environmental conditions. It has numerous uses, including as a building material, a component of concrete, an aggregate for the base of roads, filters, an ingredient for the production of lime, a supplement for farm animals, a soil conditioner, a white pigment or filler in products such as paints and toothpaste.