Onyx is a cryptocrystalline form of quartz. The colours of its bands range from white to almost every colour. Commonly, specimens of onyx available contain bands of colours of white, tan, and brown.
Pure black onyx is common, and perhaps the most famous variety, but not as common as onyx with banded colours.
It has a long history of use for hardstone carving and jewellery, where it is usually cut as a cabochon, or into beads, and is also used for intaglio or cameo engraved gems, where the bands make the image contrast with the ground.