Sardonyx is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of silicon dioxide, their full chemical compound being SiO2.
Sardonyx is a gem variety of reddish-brown colored sard with white or black banded chalcedony or onyx, used to make cameo with the raised black, red, or brown background. It is an example of a layer stone subvariety of Chalcedony, which in turn is a cryptocrystalline variety of Quartz.
Layer stones are multi-layered materials used in the art of gem-carving and engraving, also called glyptography. Usually this material is cut from agates with even parallel layers, a lighter layer above a darker one. Brazil supplies the best raw material, usually two-layered, but sometimes three-layered ones are seen. Some masterpieces are cut out of five-layered material. Engravings in multi-layered and curved agates are rare.
The combination of a black base and a white upper layer is called "onyx," different from the true onyx and onyx marble. In the case of "sard-onyx" the base is brown. While sard is dark reddish-brown, sardonyx is sard with white bands, or white with brown bands.
There are also "eye agate" varieties of banded agate, onyx, chalcedony, or sardonyx, in which the delicate colored bands are circularly arranged in such a way that they resemble an eye.
Sardonyx with varied prefixes, is usually mistaken for catalinite, and mistakenly applied to carnelian and sard.
Sardonyx is considered the gem symbol of the apostle John.
The Ring of Polycrates, a legendary ring, according to Pliny, was made of sardonyx (according to Herodotus an emerald ring) belonged to Polycrates the ruler of the Aegean isle of Samosa in the fifth century BC.
Sardonyx is often cut cabochon and tumbled. Mostly sardonyxes are carved as cameo, in which the relief being worked in the white layer with raised black, red or brown background.
A bluish-gray tint is produced by the use of the thin white upper layer, that is, by the distribution of light and the translucency of the black/brown background. Such layer stones are sometimes called "niccolo" These stones are in demand for seal rings and for engravings of coat-of-arms and initials. Engravings which have a negative picture, as used for a seal, are called "intaglios"; those with solid, raised images, are called "cameos".
Layers in agate, as required for this work, are often not found in nature in the colors of onyx, carnelian, or sard. Therefore such stones are mostly dyed. The dyed and natural stones have the same names.
Sardonyx, along with other stones with nearly flat bands, such agate, onyx, sardonyx, opal, shells, jet, rock crystal, giant conch, helmet shell, are considered suitable materials for cameos, a translucent layered gemstone generally composed of two layers (or more) and colors. The top layer has a raised figure or a design carved on it, while the second layer with other colors forms a background.
It is also used for engraved gemstones, an exquisite production of the lapidary arts of miniature sculptures, monogram, design, portrait, or inscription known as engraved gems. Most often used stones were agate and sardonyx for a two-color effect.