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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.
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.[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 |
Contents |
Etymology
Onyx comes through Latin from the Greek onyx meaning 'claw' or 'fingernail'. With its fleshtone color, onyx can be said to resemble a fingernail. The English word 'nail' is cognate. [2]
Historical usage
The Beinecke Rare Book and Manuscript Library at Yale University was originally planned to be coated in green onyx. However, there wasn't sufficient green onyx in the world to build such a structure, so that the designers used marble. Onyx was known to the ancient Greeks and Romans.[3] Use of sardonyx appears in the art of Minoan Crete, notably from the archaeological recoveries at Knossos.[4] Onyx was used in Egypt as early as the Second Dynasty to make bowls and other pottery items.[5] In folk religion, onyx is sometimes used for targets of psychic attacks of all forms, especially those sexual in nature. [6]
Precautions
If onyx is cleaned with an ultrasonic device or cleaned with abrasive or ammonia based chemicals, discoloration of the stone may occur.
Line note references
- ^ Profile of onyx
- ^ http://www.etymonline.com/index.php?term=onyx
- ^ International Colored Gemstone Association: Onyx
- ^ C. Michael Hogan, Knossos fieldnotes, Modern Antiquarian (2007)
- ^ Mary Winearls Porter, What Rome was Built with: A Description of the Stones Employed, 1907, H. Frowde, Rome, 108 pages
- ^ The Magickal Properties of Gemstones: Black Onyx
See also
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Wikimedia Commons has media related to: Onyx |
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