Travertine:

Travertine is a form of limestone deposited by mineral springs, especially hot springs.  It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. Simply put, travertine is limestone with holes in it.  Travertine tiles are commonly used in for floors and walls.  There are also many different finishes.  Brushed vs Honed.  Filled vs Unfilled.  Straight edge vs Chiseled edge.