Limestone is a sedimentary rock that is at least 50 percent calcium carbonate, and is found in nature in the form of the mineral calcite. Calcium carbonate has the molecular formula CaCO3. The formation of limestone can occur either through evaporation or the accumulation of shells from dead organisms like oysters in a marine environment.
Sedimentary rock refers to the fact that it is made up of sediment layers that are compacted. Limestone can be white or light-colored in appearance, and has a variable grain size.
There are different limestone types, including oolite, chalk and travertine. The difference among these limestone types is the conditions under which they were formed. For example, travertine forms through evaporation in cave environments. The uses of limestone include the fabrication of lime and cement through a heating process.