Limestone : A sedimentary rock consisting primarily of calcium carbonate, often in the form of the minerals calcite or aragonite, and sometimes with magnesium carbonate in the form of dolomite. Minor amounts of silica, feldspar, pyrite, and clay may also be present. Limestone can occur in many colors but is usually white, gray, or black. It forms either through the accumulation and compaction of fossil shells or other calcium-carbonate based marine organisms, such as coral, or through the chemical precipitation of calcium carbonate out of sea water: used as a building stone and in the manufacture of cement, lime, etc.
