Limestone is a sedimentary rock formed from the remains of skeletons of sea creatures. Shelly limestone and gastopod limestone contain sea shells. Oolithic limestone is made from rounded grains called ooliths. The rounded grains are formed from calcite (calcium carbonate) contained in skeletons being deposited on sand as the sand is rolled about by the action of the waves. The rock is grey in colour. Limestone is a very useful mineral. When heated it is turned to lime which is used in agriculture. it is used in making Portland cement and in smelting iron and lead.