a sedimentary rock consisting mainly of calcium carbonate, often composed of the organic remains of sea animals, as mollusks, corals, etc., and used as building stone, a source of lime, etc.: when crystallized by heat and pressure it becomes marble
See limestone in American Heritage Dictionary 4
(līmˈstōnˌ)
noun
A common sedimentary rock consisting mostly of calcium carbonate, CaCO3, used as a building stone and in the manufacture of lime, carbon dioxide, and cement.