Onyx is commercially classified as a sibling to marble because these two stones have the same mineral content. Onyx is made of calcite, which is the same mineral that makes up limestone, marble, and travertine. Onyx forms from spring water or groundwater that has an abundance of calcite dissolved in it.
Travertine is formed in the same manner as onyx. The main difference between travertine and onyx is that travertine has a porous, lace-like texture while onyx is smooth and non-porous. Aside from that, the two stones are similar.