Onyx is the striped semiprecious variety of agate with white and black alternating bands. The word onyx derives from the Greek word anux or “claw.” Other varieties of onyx are the carnelian onyx, with white and red bands, and sardonyx, with white and brown bands.
Onyx is made up of black and white bands. It is similar to agate except that the bands are straight. Black onyx has almost always been stained. Natural black onyx is rare and so it is produced by chemically treating agate. It is also called true onyx or Arabic onyx. The name onyx properly applies only to the agate variety. It is translucent to nearly opaque with a vitreous luster.
Onyx is also a name sometimes used for unicolored chalcedony. It is popular for carved cameos and intaglios because its layers can be cut to show a color contrast. Onyx must not be confused with onyx marble, which is a limestone formed by the minerals calcite and aragonite.
Onyx is found in India, Brazil and Uruguay.