Onyx
Onyx is a cryptocrystalline 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. Sardonyx is a variant 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.
The agate-like sardonyx (banded agate). The specimen is 2.5 cm (1
inch) wide. It 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 much is produced by the
staining of agate.
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