Travertine is a unique form of limestone that is increasingly popular as a coping and decking material around swimming pools. It resembles marble, but is a little softer, and has more unusual variations, including many small voids (holes) throughout the material.
As a decking material, travertine is normally cut into rectangular pavers. For a less formal look, you may want to consider tumbled travertine. These rectangular pavers have been literally tumbled in a bin to round off most of the sharp corners, and provide a somewhat antiqued look.