Onyx, like Travertine, is the result of water dissolving existing limestone and re-depositing it as a new kind of stone, sometimes called sinter. In Limestone caves, onyx is formed by drip water, as stalagmites and stalactites. Onyx is a very soft stone, and somewhat brittle, and needs to be installed where it will not be subject to areas of hard ware.  This beautiful stone is characterized by its translucence, and can actually be backlit for striking, dramatic effects.