Travertine, a form of limestone, is a sedimentary rock and composed of calcium carbonate. It is formed along streams -particularly where there are waterfalls- and around hot or cold springs. Travertine is characterised by pitted holes and troughs in the surface, although it can be purchased filled or unfilled. A large quantity of travertine was used to construct The Coliseum in Rome and many other well-known buildings also feature it. It can be honed or polished to a smooth, shiny finish and comes in many shades of white through to cream, beige and red. Like all natural stone, travertine should be sealed to give it added stain resistance and to aid cleaning. As it is acid-sensitive, travertine should be cleaned with a neutral ph or slightly alkaline cleaner.