Onyx is a typically black-and-white banded variety ofagate, asilicate mineral. The bands can also be monochromatic with alternating light and dark bands.Sardonyx is a variety with red to brown bands alternated with black or white bands. The name "onyx" is also frequently used forlevel-banded (parallel-banded) agates, but in proper usage it refers to color pattern not band structure.[1] Onyx, as a descriptive term, has also been incorrectly applied to parallel-banded varieties ofalabaster,marble,calcite,obsidian, andopal, and misleadingly to materials with contorted banding, such as "cave onyx" and "Mexican onyx".[1][3][4]
Onyx comes through Latin (of the same spelling), from the Ancient Greekὄνυξ (onyx), meaning'claw' or'fingernail'. Onyx with pink and white bands can sometimes resemble a fingernail.[5] The English word "nail" iscognate with the Greek word.
Cabochons of red onyx, also called sardonyxBlack onyx with white streaks
Onyx is formed ofchalcedony bands in alternating colors. It iscryptocrystalline, consisting of fine intergrowths of thesilica mineralsquartz andmoganite. Its bands are parallel, unlike the more chaotic banding that often occurs inagates.[6]
Sardonyx is a variant in which the colored bands aresard (shades of red) rather than black. Black onyx is perhaps the most famous variety, but it is not as common as onyx with colored bands. Several artificial treatments have been used since ancient times to produce the black color in "black onyx" and the reds and yellows in sardonyx. Most "black onyx" on the market is artificially colored.[7][8]
The name has also commonly been used to label other banded materials, such as bandedcalcite found inMexico,India, and other places, and often carved, polished, and sold. This material is much softer than true onyx and more readily available. The majority of carved items sold as "onyx" today are thiscarbonate material.[9][10]
Artificial onyx types have also been produced from common chalcedony and plain agates. The first-century naturalistPliny the Elder described these techniques used in Roman times.[11] Treatments for producing black and other colors include soaking or boiling chalcedony in sugar solutions, then treating withsulfuric orhydrochloric acid to carbonize sugars which had been absorbed into the top layers of the stone.[8][12] These techniques are still used, as well as other dyeing treatments, and most so-called "black onyx" sold is artificially treated.[13] In addition to dye treatments, heating and treatment withnitric acid have been used to lighten or eliminate undesirable colors.[8]
Onyx can be found in various regions of the world, including Mexico, Greece, Yemen, Uruguay, Argentina, Australia, Brazil, Canada, China, Czech Republic, Germany, Pakistan, India, Indonesia, Madagascar, the UK, and various states in the US.[9]
TheGemma Augustea is a Romancameo produced 9–12 AD andcarved in a two-layered onyx gem (19 × 23 cm).
It has a long history of use forhardstone carving andjewelry, where it is usually cut as acabochon or into beads. It has also been used forintaglio and hardstonecameoengraved gems, where the bands make the image contrast with the ground.[14] Some onyx is natural but much of the material in commerce is produced by the staining of agate.[15]
Onyx was used inEgypt as early as the Second Dynasty to make bowls and other pottery items.[16] Use of sardonyx appears in the art ofMinoanCrete, notably from the archaeological recoveries atKnossos.[17]
Brazilian green onyx was often used as plinths forart deco sculptures created in the 1920s and 1930s. The German sculptorFerdinand Preiss used Brazilian green onyx for the base on the majority of hischryselephantine sculptures.[18] Green onyx was also used for trays and pin dishes—produced mainly inAustria—often with small bronze animals or figures attached.[19]
Onyx is mentioned in the Bible many times.[20] Sardonyx (onyx in which white layers alternate with sard—a brownish color) is mentioned in the Bible as well.[21]
The ancient Romans entered battle carrying amulets of sardonyx engraved withMars, the god of war. This was assumed to bestow courage in battle. In Renaissance Europe, wearing sardonyx was assumed to bestow eloquence.[25] A traditionalPersian assumption is that it helped withepilepsy.[26] Sardonyx was traditionally used by English midwives to ease childbirth by laying it between the breasts of the mother.[27]
^Assaad, Fakhry A.; LaMoreaux, Philip E. Sr. (2004). Hughes, Travis H. (ed.).Field Methods for Geologists and Hydrogeologists. Berlin,Heidelberg, New York:Springer-Verlag. p. 8.ISBN3-540-40882-7.
^abO'Donoghue, Michael (1997).Synthetic, Imitation, and Treated Gemstones. Boston: Butterworth-Heinemann. pp. 125–127.ISBN0-7506-3173-2.
^Read, Peter G. (1999).Gemmology. Oxford: Butterworth-Heinemann. p. 160.ISBN0-7506-4411-7.
^Liddicoat, Richard Thomas (1987).Handbook of Gem Identification (12th ed.). Santa Monica, California: Gemological Institute of America. pp. 158–160.ISBN0-87311-012-9.
^Kraus, Edward Henry; Slawson, Chester Baker (1947).Gems and Gem Materials. New York, New York: McGraw-Hill. p. 227.
^Liddicoat, Richard Thomas; Copeland, Lawrence L. (1974).The Jewelers' Manual. Los Angeles, California: Gemological Institute of America. p. 87.
