- Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as corals, foramen and mollusks. Its main materials are calcite and aragonite minerals, which are different crystalline forms of calcium carbonate (CaCO3). About 10% of the sedimentary rocks are limestone. The solubility of limestone in water and solutions of weak acid lead to karstic landscapes, in which water erodes limestone over thousands to millions of years. Most cave systems pass through the limestone rock.