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Onyx is a cryptocrystalline form of quartz. Its bands of color range from white to almost any color (some with some shades of another color like purple or blue); common colors include white, tan, and brown. Pure black onyx is common, but not as common as the banded variety.
The planet Onyx had such plentiful deposits of onyx that it was named for the mineral. Zone 67 on Onyx was also home to a Forerunner city built primarily of onyx. The Covenant also use onyx as a decorative material.
Onyx is usually cut as a cabochon, or into beads, and is also used for intaglios and cameos, where the bands make the image contrast with the ground. Some onyx is natural but the staining of agate produces much.
The name 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 are this carbonate material.
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