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Onyx is a form of quartz. The colors of its bands range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of onyx available contain bands of colors of white, tan, and brown. The word "onyx" comes through Latin ONYX, from the Greek word ONYX meaning "claw" or "fingernail". With its flesh tone color, onyx can be said to resemble a fingernail.

The name "onyx" has sometimes been used, incorrectly, to label other banded lapidary materials, such as banded calcite found in Mexico, Pakistan, and other places, and often carved, polished and sold. This material is much softer than true onyx, and much more readily available. The majority of carved items sold as "onyx" today is this carbonate material. The best onyx is found in the Arabian Peninsula, Brazil, Uruguay, and the USA.

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Onyx has a variety called sardonyx in which the colored bands are sard (shades of red) rather than black. Pure black onyx is common, and perhaps the most famous variety, but not as common as onyx with banded colors.

Onyx has a long history of use for hardstone carving and jewelry, 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. Some onyx is natural but much is produced by the staining of agate.

Onyx was known to the Ancient Greeks and Romans. Use of sardonyx appears in the art of Minoan Crete, notably from the archaeological recoveries at Knossos. Onyx was used in Egypt as early as the Second Dynasty to make bowls and other pottery items. Onyx is also mentioned in the Bible at various points, such as the priests' garments and the foundation of the city of Heaven in Revelation.