Sedimentary rock composed mainly of calcium carbonate (CaCO 3 ), usually in the form of calcite and, less commonly, aragonite .
It may contain considerable amounts of magnesium carbonate ( dolomite ) as well. Most limestones have a granular texture; in many cases, the grains are tiny fragments of fossil animal shells. Much knowledge of the Earth's history has been derived from the study of fossil s embedded in limestone and other carbonate rocks. Limestone is used as a soil conditioner, in the manufacture of glass, and in agriculture. Ornamental varieties are used for flooring, exterior and interior facings of buildings, and monuments.