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Sardonyx (banded agate).
The specimen is 2.5 cm (1 inch) wide.
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 has a long history of use for hardstone carving and jewellery, where it is usually cut as a cabochon, or into beads, and is also used for intaglio or cameo engraved gems, 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 Mexicomarker, Pakistanmarker, 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.

Technical details
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


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 with the Greek word.

Historical usage

Onyx was known to the ancient Greeks and Romans. Use of sardonyx appears in the art of Minoan Cretemarker, notably from the archaeological recoveries at Knossosmarker. Onyx was used in Egyptmarker as early as the Second Dynasty to make bowls and other pottery items.

Black onyx with bands of colors.


References

  1. Profile of onyx
  2. http://www.etymonline.com/index.php?term=onyx
  3. International Colored Gemstone Association: Onyx
  4. C. Michael Hogan (2007) Knossos fieldnotes, The Modern Antiquarian
  5. Mary Winearls Porter, What Rome was Built with: A Description of the Stones Employed, 1907, H. Frowde, Rome, 108 pages


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