Onyx, a variety of chalcedonic quartz. It differs from agate in that the bands, or layers, of different colors are straight and parallel instead of curved. Onyx usually has red or brown bands that alternate with white, but black is sometimes found. Since the bands are wide and even, onyx is often used to make cameos and intaglios. When reddish-brown bands alternate with white or black, the stone is usually called sardonyx. Most commercial onyx, such as black onyx and blue onyx, is not onyx but another form of chalcedony that has been dyed. Mexican onyx and onyx marble are not true onyxes but are calcite types of alabaster.

Onyx was used by the ancient Greeks and Romans in vases and cups and in rings, cameos, and other jewelry.