Limestone is a sedimentary rock that is usually white in color, as it is largely composed of calcium carbonate. Limestone can be colored by impurities, however; iron oxide can make limestone red, brown or yellow, while carbon can make it gray, black or blue.
Limestone is known by many other names, which are determined by the rock's appearance, how it was formed, its composition and other factors. Common names for limestone include:
- Chalk - Chalk is a soft limestone that is typically light gray or white, and it has a very fine texture. The calcareous shell remains of many different microscopic marine organisms form chalk.
- Travertine - Travertine forms in caves from evaporated precipitation.
- Coquina - Coquina is formed on beaches from broken shell debris.
- Lithographic limestone - Lithographic limestone typically features uniform and fine grain sizes, and it is quite dense. The grains form in thin beds, which are easy to separate, resulting in a very smooth surface.
- Tufa - The precipitation of calcium-rich waters, such as hot springs, lake shores and other areas, produces tufa limestone.
- Fossiliferous limestone - Fossiliferous limestone is formed from shell and skeletal fossils.
- Oolitic limestone - Oolitic limestone is largely composed of calcium carbonate oolites, which are spheres that form from concentric precipitation on shell fragments or sand grains.