Travertine is a form of limestone deposited by mineral springs, especially hot springs. It is sometimes known as travertine limestone or travertine marble; these are the same stone, although travertine is classified properly as a type of limestone, not marble. The stone is characterised by pitted holes and troughs in its surface. Although these troughs occur naturally, they suggest signs of considerable wear and tear over time. Some installers use a grout to fill the holes, whereas others leave them open — travertine can be purchased "filled" or "unfilled." It can also be polished to a smooth, shiny finish, and comes in a variety of colors from grey to coral-red. Slabs can be selected at a stone yard and reserved by the consumer prior to fabrication.