Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal, and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water. Limestone is a rock with an enormous diversity of uses. It could be the one rock that is used in more ways than any other. Most limestone is made into crushed stone and used as a construction material. It is used as a crushed stone for road base and railroad ballast. It is used as an aggregate in concrete. It is fired in a kiln with crushed shale to make cement. Some varieties of limestone perform well in these uses because they are strong, dense rocks with few pore spaces. These properties enable them to stand up well to abrasion and freeze-thaw. Although limestone does not perform as well in these uses as some of the harder silicate rocks, it is much easier to mine and does not exert the same level of wear on mining equipment, crushers, screens, and the beds of the vehicles that transport it.