Limestone is a sedimentary rock consisting mostly of the mineral calcite (calcium carbonate). The primary source of this mineral is the remains of marine animals. These organisms secrete calcium carbonate which forms their protective shells. Upon their death the shells break apart and dissolve. The accumulation and recrystallization of seashells cements other sediments together upon drying, forming limestone. Calcium carbonate may be derived from organic sources as well as inorganic sources.
Although fossils are frequently visible in limestones, the shells responsible for some deposits have gone through enough change that they are no longer identifiable. For example, crystalline calcium carbonate forms the stone commonly called marble, which is prized for its beauty and extraordinary hardness. Dissolved calcite may also be deposited by supersaturated waters that precipitate forming stalactite and stalagmite deposits common to caves.
It is thought that the majority of the Earth's sedimentary rocks were deposited during the Biblical deluge. Vast limestone deposits can be found that reach hundreds of square miles. These sites have historically been mined to obtain the limestone for use in building stone and in the manufacture of lime and cement. In antiquity, the Egyptians commonly used limestone for the construction of pyramids and other monuments. The pyramids at Giza and the great Sphinx, for example, were built almost entirely out of limestone. Even in modern times, limestone continues to be a valued commodity for building material. The U.S. Geological Survey reports that the U.S. produced 1.65 billion tons of crushed stone in 2005, of which 70% was limestone .