Onyx refers to a black and white banded variety of agate. Brown varieties are named sardonyx. It is composed of relatively straight, parallel layers of different colours.

It is usually cut into cabochon (convex hemispherical or oval form, polished but not cut into facets) or into beads, and is also used for intaglios and cameos. Some onyx is natural, but most is produced by the staining of agate. It is available in the USA, Germany, Brazil, Mexico, India, and Africa.

Sources: Encyclopedia of gemstones and minerals; Random House Websters' Unabridged Dictionary