As one of the most interesting stones used in the decorative art of stonework, travertine is a stone that is very common in Italy and is a sedimentary stone that is a type of limestone. The interesting thing about this type of stone is that it must be placed under significant pressure for several thousands of years in order to turn from standard limestone into travertine. This is the same sort of process wherein peat turns into coal. Further exposure to significant pressure will then turn travertine into marble. Ancient Romans used all types of stone in their amazing architecture.