onyx

 

(ŏn'ĭks) , variety of cryptocrystalline quartz, differing from agate only in that the bands of which it is composed are parallel and regular. Its appearance is most striking when the bands are of sharply contrasting colors; black and white specimens are often used for cameos. Onyx was used in Roman times for the fabrications of vases and cups. Sardonyx contains onyx and carnelian or sard. “Onyx marble,” “Mexican onyx,” and “Oriental alabaster” are terms applied to travertine.

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]

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

Culture and historical usage

History

The word Shoham in the Hebrew Bible (including in the verse Exodus 28:20), which describes a stone in the Hoshen and two stones on the shoulder straps of the Ephod, means onyx, according to Rebbenu Bachya, and additionally represents the Tribe of Joseph. The Septuagint mostly translates the term as beryl, but in some places translates it as onyx, and in a couple of places translates it as leek-green stone; scholars think (according to sources like the Jewish Encyclopedia) that malachite is the most likely meaning since it is banded like onyx, and green like beryl and leek-green stones, and cloudy as beryl can sometimes appear. There is a wide range of views among traditional sources about which tribe the stone refers to.

The word Yahalom in the Hebrew Bible is also thought to be an Onyx by some. The Septuagint usually translates it as an Onyx, but sometimes as a beryl or as a jasper. English translations usually follow one of the Septuagint's choices, but vary as to which. Onyx only started being mined after the Septuagint was written, so the Septuagint's term Onyx probably doesn't mean Onyx; Onyx is originally an Assyrian word meaning ring, and so could refer to anything used for making rings. Yahalom is similar to a Hebrew word meaning hit hard, so some people think that it means diamond. Scholars think that the word refers to Sardonyx - red Onyx, because the sequence of stones in a wall in the Book of Revelation is similar to the Septuagint's list of stones in the Hoshen, which has Sardonyx where the Yahalom stone would be expected.

In the Dreamlands, a fictional location in the works of the author H. P. Lovecraft, the palaces of Kadath are built of onyx.

Astrological Relations

The onyx is the stone for the zodiac sign, Leo. It can be many different colors, but the ones with the reddish brown color is the preferred type. Onyx is said to banish grief and bring fortune to the possessor of it. It's said to also bring recognition of personal strengths.

Precautions

Onyx is not meant to be cleaned with an ultrasonic cleaner or cleaned with abrasive or ammonia based cleaner as using such types can cause discoloration of the stone.

References

• http://www.gemstone.org/gem-by-gem/english/onyx.html
• http://www.mindat.org/min-2999.html

See also

• List of minerals

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