Limestone:

Common type of sedimentary rock composed principally of calcite (calcium carbonate, CaCO3). When "burned" or calcined (raised to a high temperature), it yields lime (calcium oxide, CaO). Crystalline metamorphosed limestone is known as marble. Many varieties of limestone are formed by the consolidation of sea shells, which are formed by the largely CaCO3 secretions of various marine animals. Chalk is a variety of porous, fine-grained limestone composed mostly of foraminifera shells; coquina is a soft limestone made up of shell fragments. A variety of the rock, known as oölitic limestone, is composed of small egg-shaped concretions, each containing a nucleus of a sand grain or other foreign particle around which deposition has taken place. Some types of limestone, such as Portland stone, are used in building.