Travertine often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. It is formed by a process of rapid precipitation of calcium carbonate. Travertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from solution in ground and surface waters, or geothermally heated hot-springs. Travertine is often used as a building material. The Romans mined deposits of travertine for building temples, aqueducts, monuments, bath complexes, and amphitheaters such as the Colosseum. It can also be polished to a smooth, shiny finish, and comes in a variety of colors from grey to coral-red. Travertine is most commonly available in tile sizes for floor installations. Travertine is one of the most frequently used stones in modern architecture. It is commonly used for façades, wall cladding, and flooring.