True onyx is a fine-grained siliceous gemstone, but the term has been used for calcareous marble-like layered sediments, technically termed travertine. This onyx is typically deposited from solutions in ground and surface waters, often in caves. The often colorful and banded surfaces, when polished, are sought after for their beauty.
Travertine is found in many parts of the world including the US. Deposits of travertine in Mexico are also being used for cutting and carving into many beautiful objects. Click her for more information on MEXICAN ONYX.