Limestone is a sedimentary rock composed mainly of the mineral calcite, or calcium carbonate (CaCO3). In its natural form it comprises 15% of the earth's crust, and is extensively used in the building industry, being a vital ingredient in aggregates such as cement and lime, and represents 71% of the crushed rock used in construction. If heated, limestone can be decomposed to form lime (with a 56% yield), which is used in steel manufacturing, road construction and paper production.
Limestone forms either directly from water (the calcium and carbonate ions react and may crystallise if variances in temperature are high enough) or from the accumulation of shell fragments. This usually occurs in water of less than 20m in depth. It is mainly composed of calcite but also contains aragonite (CaCO3 with a different structure) and dolomite (CaMg(CO3)2). In its pure form it will appear white, however will take on a wide range of colours if impurities are present. If exposed to intense pressure and heat limestone will undergo metamorphosis to form marble.