Onyx

Onyx is a typically black-and-white banded variety of agate, a silicate 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 for level-banded (parallel-banded) agates, but in proper usage it refers to color pattern not band structure. Onyx, as a descriptive term, has also been incorrectly applied to parallel-banded varieties of alabaster, marble, calcite, obsidian, and opal, and misleadingly to materials with contorted banding, such as "cave onyx" and "Mexican onyx".

Onyx

General

Category Tectosilicate minerals, quartz group, chalcedony variety, agate variety

Formula SiO2 (silicon dioxide)

Crystal system Trigonal (quartz), Monoclinic (moganite)

Identification

Formula mass 60.08 g/mol

Color Black and white; red to brown with black or white (sardonyx)

Cleavage None

Fracture Uneven, conchoidal

Mohs scale hardness 6.5–7

Luster Vitreous, silky

Streak White

Diaphaneity Translucent

Specific gravity 2.55–2.70

Optical properties Uniaxial/+

Refractive index 1.530–1.543

Etymology

Onyx comes through Latin (of the same spelling), from the Ancient Greek ὄνυξ (onyx), meaning 'claw' or 'fingernail'. Onyx with pink and white bands can sometimes resemble a fingernail. The English word "nail" is cognate with the Greek word.

Properties

Chalcedony and thus also the varieties agate and onyx are in turn micro- to cryptocrystalline varieties of themineral quartz(SiO2). As Onyx are the names given to special colour varieties of chalcedony oragate. By definition, purely black chalcedony and black and white banded agate are considered to be onyx. If the mineral occurs as an unstructured mass, it is called chalcedony; if it consists of clearly visible, differently coloured layers or bands, it is called agate. 

To distinguish them from onyx, uniformly brown (in antiquity and later also yellow-red to red) chalcedony is called carnelian (also sardine) and black-white-red to reddish-brown banded chalcedony (agates) is called sardonyx. Like chalcedony and agate, onyx is opaque to slightly translucent and is found in nature predominantly as fine-grained, porous mineral aggregates in grape-like to nodular or stalactite forms, often with a fibrous appearance. 

Since onyx is not found frequently enough to meet demand, it is often imitated by appropriately colored chalcedony and agate. The art of dyeing agate was already known to the Romans. Confusion with other black minerals such as obsidian, schorl, or jet is also possible. Banded flint in shades of white, grey and black is distinguishable from quartz due to its almost identical consistency and softer banding.

Also confused with onyx is onyx marble (also known as onyx marble or marble onyx), which is often misleadingly abbreviated to onyx. Onyx marble, however, is a yellow, brown, or green limestone sinter rock composed of either calcite (CaCO3) or aragonite (CaCO3). An important distinguishing feature, besides its lower hardness, is its sensitivity to acids. Genuine onyx is primarily used to make gemstones or craft utensils, such as letter openers, ashtrays, etc. Onyx stones were popular as jewelry in the first half of the 20th century, especially in rings and cufflinks.

Varieties

Onyx is formed of chalcedony bands in alternating colors. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and moganite. Its bands are parallel, unlike the more chaotic banding that often occurs in agates.

Sardonyx is a variant in which the colored bands are sard (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. 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.[better source needed][disputed – discuss]

There are several varieties of onyx:

Green onyx: Translucent and green in tone, the darker shade being the most prized. It often has brown veins.

San Martin Onyx: opaque, with brown and reddish hues. It presents bands of varying intensity, and its extreme hardness allows for perfect polishing.

Onixtin: onyx that has materials from both varieties.

Aragonite: whitish or greyish in colour, where the fibres are thicker than in the previous varieties.

Sardonyx: A variant in which the color bands are sardinian (shades of red) instead of black.

Black onyx: Perhaps the most famous variety, but not as common as colored banded onyx. Artificial treatments have been used since ancient times to produce both the black color in "black onyx" and the reds and yellows in sardonyx. Most "black onyx" on the market is artificially colored, although in rare cases it may be genuine black onyx.

Geographic occurrence

Onyx can be found in various regions of the world, including Greece, Yemen, Uruguay, Argentina, Australia, Brazil, Canada, China, Czech Republic, Germany, Pakistan, India, Indonesia, Madagascar, Latin America, the UK, and various states in the US.

Use as a gemstone

Its deep black color makes onyx a popular gemstone, especially for mourning occasions. It is also often used for carving gems. Onyx is also frequently used to make cufflinks, especially for black tie occasions (tuxedoes, cutaways).

Historical use

It has a long history of use for hardstone carving and jewelry, where it is usually cut as a cabochon or into beads. It has also been used for intaglio and hardstone cameo engraved gems, where the bands make the image contrast with the ground. Some onyx is natural but much of the material in commerce is produced by the staining of agate.

Onyx was used in Egypt as early as the Second Dynasty to make bowls and other pottery items. Use of sardonyx appears in the art of Minoan Crete, notably from the archaeological recoveries at Knossos.

Brazilian green onyx was often used as plinths for art deco sculptures created in the 1920s and 1930s. The German sculptor Ferdinand Preiss used Brazilian green onyx for the base on the majority of his chryselephantine sculptures. Green onyx was also used for trays and pin dishes – produced mainly in Austria – often with small bronze animals or figures attached.

Onyx is mentioned in the Bible many times. Sardonyx (onyx in which white layers alternate with sard – a brownish color) is mentioned in the Bible as well.

Onyx was known to the Ancient Greeks and Romans. The first-century naturalist Pliny the Elder described both types of onyx and various artificial treatment techniques in his Naturalis Historia.

Slabs of onyx (from the Atlas Mountains) were famously used by Mies van der Rohe in Villa Tugendhat at Brno (completed 1930) to create a shimmering semi-translucent interior wall.

The Hôtel de la Païva in Paris is noted for its yellow onyx décor, and the new Mariinsky Theatre Second Stage in St.Petersburg uses yellow onyx in the lobby.

Imitations and treatments

Due to the high demand, which cannot be met by natural deposits, a large proportion of the onyx available on the market is fake. Either the porous agate (cf. Greek-Latin achates onyx) is dyed or gray layers of chalcedony are immersed in an acid bath. The name has also commonly been used to label other banded materials, such as banded calcite found in Mexico, 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 this carbonate material.

The pickling takes place by placing it in a sugar or honey solution and then heating it to high temperatures, converting the sugar into carbon, which is then fixed by treatment with heated sulfuric acid. Sometimes shiny black obsidian is polished as an imitation. The result is a matte, deep black color. Distinguishing real from fake onyx is extremely difficult and can basically only be done through mineralogical analysis. However, if a stone is streaked with white, it can be assumed to be a genuine specimen.

Artificial onyx types have also been produced from common chalcedony and plain agates. The first-century naturalist Pliny the Elder described these techniques used in Roman times. Treatments for producing black and other colors include soaking or boiling chalcedony in sugar solutions, then treating with sulfuric or hydrochloric acid to carbonize sugars which had been absorbed into the top layers of the stone. These techniques are still used, as well as other dyeing treatments, and most so-called "black onyx" sold is artificially treated. In addition to dye treatments, heating and treatment with nitric acid have been used to lighten or eliminate undesirable colors.

Superstitions

The ancient Romans entered battle carrying amulets of sardonyx engraved with Mars, the god of war. This was believed to bestow courage in battle. In Renaissance Europe, wearing sardonyx was believed to bestow eloquence. A traditional Persian belief is that it helped with epilepsy. Sardonyx was traditionally used by English midwives to ease childbirth by laying it between the breasts of the mother.

Esotericism

Already in ancient times, but also in the Middle Ages, various precious and semi-precious stones were associated with known planets and constellations and used as amulets or healing stones with supposed magical properties. This type of use also continues in esotericism, but the assignment is purely arbitrary and differs from author to author. For example, according to Richardson and Huett (1989), onyx and sardonyx are assigned to Mars; according to Ahlborn (1996), sardonyx is assigned to Jupiter; and according to Uyldert (1983), onyx is assigned to Saturn. 

Likewise, a variety of zodiac stones are in use for the signs of the zodiac. Depending on the source, onyx is mainly associated with Capricorn, but also with Gemini or Leo, and sardonyx with either Virgo or Libra. Since the zodiac stones were and are also considered birthstones, many gemstones are also assigned to specific months. However, the connection is arbitrary and is rarely related to the corresponding zodiac signs. This is particularly evident with onyx, which, depending on the source, is assigned to February or August (Capricorn December 22nd–January 20th; Gemini May 21st–June 21st; Leo July 23rd–August 23rd). 

According to various esoteric texts, onyx is said to be a healing stone that can relieve or cure eye infections and toenail diseases, among other things. There is no scientific evidence for its alleged healing properties. Hildegard von Bingen lists numerous ailments for which onyx can be used, including hand pain, eye diseases, fever, spleen pain, and stomach pain. It can also relieve sadness simply by looking at it or putting it in the mouth. 

Sourced from Wikipedia