Nearly all examples of limestone begin their life as live sea creatures. In the case of chalk they are microscopic protozoa (single cell animals). Coral is composed of the skeletons of tiny multicellular animals. A limestone called coquina is composed of millions of sea shells and has a much coarser look. Travertine and tufa are crystalline deposits that form from the evaporation of dissolved calcium carbonate in caverns or hot springs. Over millions of years these deposits are compressed to form limestone, which can be fine or coarse grained, depending on the origin.
One route that the life cycle of limestone can take is its metamorphosis into marble. It is formed over millions of years when limestone is subjected to intense pressure. Marble is usually a whitish color, but can often be colored with impurities. It is possible to make out tiny pieces of calcite when marble is studied closely. Compacted crystalline limestone can sometimes be mistaken for marble.
Most limestone ends its life just lying in the ground as a sedimentary rock. However, it has been mined for centuries for building blocks. The Great Pyramid of Giza is made entirely from limestone, as are most of the artifacts that make up the tombs, statues and temples of ancient Egypt. Limestone can be seen all over North America and Europe as the material used for public buildings, and in medieval buildings and castles in Europe.
Pulverized limestone is the raw material used in the manufacture of quicklime, slaked lime and cement. The same pulverized material is used to treat acidic soils and as an aggregate for the solid base of many roads. It is used in blast furnaces to extract iron from iron ore. The mineral is also used as a constituent of medicines and cosmetics, in cereals and bread as a source of calcium, and in paper, toothpaste, tiles, paint and plastics as a pigment and filler.