Travertine is a type of limestone, but differs from other forms in that it is formed in hot springs called karst. The water movement in these karst erodes the travertine, creating holes in the stone. Polished travertine will usually have its holes filled in with colored portland cement. Since the fillers typically do not take a polish, they tend to give the stone a blotchy appearance. Unfilled travertine, which does exhibit these holes, is widely used on walls, floors and building exteriors. Travertine is commonly tan or beige in color, but can also be found in silvers and reds.