Onyx
Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz. The colors of its bands range from white to almost every color (not including shades such as purple, blue, or black). Commonly, specimens of onyx available contain colors of white, tan, and brown. Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black.
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.
One can view a close variant of this material in St.Mary's Church at Żebbuġ, Gozo.
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. [1]
| Chemical composition and name | SiO2 - Silicon dioxide |
| Hardness (Mohs scale) | 7 |
| Specific gravity | 2.65 - 2.667 |
| Refractive index (R.I.) | 1.543 - 1.552 to 1.545 - 1.554 |
| Birefringence | 0.009 |
| Optic sign | Positive |
| Optical character | Uniaxial |
Culture and historical/mythical usage
According to Rebbenu Bachya, the word Shoham in the verse Exodus 28:20 means "Onyx" and was the stone on the Ephod representing the Tribe of Joseph. There were also two Shoham stones on the shoulders of the Ephod.
In the Dreamlands, a fictional location in the works of the author H. P. Lovecraft, the palaces of Kadath are built of onyx.
See also
