Limestones are sedimentary rocks composed primarily of calcium carbonate, a mineral that comes from the shells of marine organisms. There are many different synonyms for limestones, each describing a slightly different type of this rock. Some of the most common synonyms for limestones include calcite, chalk, marble, travertine, and fossiliferous limestone. Calcite refers to a specific crystal form of calcium carbonate that can be found in limestone rocks. Chalk, on the other hand, is a soft, white limestone that is formed from the accumulation of tiny marine organisms. Marble is a metamorphic rock that is formed from limestone under intense heat and pressure. Travertine is a porous limestone that is often used for building materials. And fossiliferous limestone is a type of limestone that contains numerous fossils of ancient marine creatures.