Limestone is a sedimentary rock whose chief mineral component is calcite (calcium carbonate: CaCO3). Limestone can be formed by precipitation of calcite dissolved in water or by depostion of marine organisms and entrainment of secondary minerals. Approximately 80 to 90% of limestone composition are skeletal fragments of marine organisms such as coral or foraminifera.
Some other carbonate grains comprising limestones are soil types such as ooids, peloids, intraclasts, and extraclasts; moreover, certain limestones do not consist of grains at all, but rather and are formed completely by the chemical precipitation of calcite or aragonite, the latter also known as travertine.
Due to the ease of dissolution and precipitation processes of calcium carbonate, limestone occurrences are linked to fascinating topographic phenomena of cave, karst and limestone pavements, the latter often called alvar.