Travertine is a form of limestone without the presence of shells deposited at hot springs usually near a limestone cave. The presence of bacteria and organisms that colonise the surface of limestone is what gives it its distinctive porosity. Best used for walls unfilled to showcase nature's art, travertine can also be used filled and polished for a very classy look. Onyx is a parallel banding of silicon dioxide or chalcedony which is also what constitutes modern day glass, hence giving its distinctive semi translucency. Onyx is best used back-lit to add a big impact and to draw attention.