Limestone

From Environmental Technology

Limestone is a sedimentary rock composed largely of the mineral Calcite (i.e. Calcium Carbonate). Limestones often contain variable amounts of silica in the form of chert or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock.

[edit] Reference:

http://en.wikipedia.org/wiki/Limestone