Limestone is a type of rock made up mostly of calcite, a mineral form of calcium carbonate. Most limestone is grey, but all colours of limestone from white to black have been found.
All limestone are formed when the calcium carbonate crystallises out of solution. It leaves the solution in many ways, and each way produces a different kind of limestone.
Limestone can be formed almost completely without the aid of organisms. This type of limestone is forced out of solution when the water evaporates.
Evaporation of water in limestone caverns forms another variety of limestone, called travertine, into stalactites and stalagmites.
Some limestone can be formed by the work of organisms. Many aquatic organisms draw calcium carbonate out of the water and use it to make their shells and bones. The oysters, clams, snails, corals, and sea urchins do this. When the animals die the shells and bones are broken up by waves into shell and coral sand and mud.
Limestone makes an excellent building stone because it can be carved easily. Some factories use limestone to clean waste gases and water before releasing them into the environment. Limestone is also used to make lime and to smelt iron ore.