Onyx, hard form of quartz made up of extremely small crystals. Onyx can be recognized by its regular and straight parallel bands of white, black, or brown. The regular banding of onyx distinguishes it from agate, a virtually identical mineral. Black-and-white onyx is a popular material for cameos. Sardonyx, which is commonly used as a gemstone, is a red-brown variety of onyx with white or black bands.