Jet in gemstone

Jet is a type of lignite, the lowest rank of coal, and is a gemstone. Unlike many gemstones, jet is not a mineral, but is rather a mineraloid. It is derived from wood that has changed under extreme pressure. Jet (from the Latin gagâtes, stone of Gages, in Lycia ) is a fossil gem formed by the chemical interaction at high temperature and pressure of brackish water with a sedimentary rock rich in organic matter from the fossilized remains of plants. This variety of fossil lignite is combustible, hard, compact, glassy and shiny black, and can be treated as a fine stone , cut into facets and polished; it is used in particular for the making of jewelry, mourning sets, trimmings, and tableware .

Jet is either black or dark brown, but may contain pyrite inclusions which are of brassy colour and metallic lustre. The adjective "jet-black", meaning as dark a black as possible, derives from this material. "Jet" or "jet black" traditionally refers to a shiny black, often with metallic blue highlights, this term is used to describe the plumage of crows and their very dark black hair.

Jet is an amorphous material; its refractive index is about 1.66. Its specific gravity is about 1.3 (relative to water), and its luster is waxy to vitreous. It gives off a characteristic smell of burnt coal when a red-hot needle is applied to it. Its line is brown, which can distinguish him from his imitations.

In the 17th and 18th centuries, jet working was one of the dominant craft and industrial activities in the Pays d'Olmes, in the French department of Ariège, where several "jet gemstone mills" operated using hydraulic power.

Jet (gemstone)

General

Category Mineraloid

Formula Variable, but rich in carbon

Identification

Color Black, occasionally brown

Cleavage None

Fracture Conchoidal

Tenacity Brittle

Mohs scale hardness 2.5–4.0

Streak Brown

Specific gravity 1.3–1.4

Optical properties Isotropic

Refractive index 1.640–1.680

Dispersion None; opaque

Ultraviolet fluorescence None

Common impurities Iron, sulfur

Origin

Jet is a product of decomposition of wood from millions of years ago, commonly the wood of trees of the family Araucariaceae. Jet is found in two forms, hard and soft. Hard jet is the result of carbon compression and salt water; soft jet may be the result of carbon compression and fresh water. Despite the name they both occupy the same area of the Mohs scale with the difference being that soft jet is more likely to crack when exposed to changes in temperature.

Properties

Jet is around 75% carbon and 12% oxygen with sulfur and hydrogen making up most of the balance. Other elements are found at trace level and the exact ratios varying with the source; for example, Spanish jet contains more sulfur than Whitby jet. Jet has a Mohs hardness ranging between 2.5 and 4 and a specific gravity of 1.30 to 1.34. The refractive index of jet is approximately 1.66. The touch of a red-hot needle should cause jet to emit an odour similar to coal.

Jet is compact, smooth to the touch, light and fairly soft (around 2.35 and 6.5 on theMohs scale), you havea conchoidal fractureand a dark brown stripe colour. It burns producing a lot ofsmoke, giving off a bituminous and sometimes fetid odor. Its density ranges between 1.2 and 1.3 g/cm³. It is formed by a heterogeneous mixture of organiccarbonaceousmaterial and mineral matter. The main component isvitrinite, an organic compound derived fromlignin,celluloseand other compounds present invascular plants with seeds. Jet may induce an electric charge like that of amber when rubbed.

Jet is very easily cut using carving tools, but small pieces tend to break off, making it difficult to create fine details. It therefore takes an experienced lapidarist to execute more elaborate carvings. It is a very fragile material, so its extraction has always been artisanal, making it difficult to carve figures with abundant detail and openwork. This circumstance has given the art of jet carving limited expressive scope. It is worked with a file and lathe, and through proper polishing, it acquires an intense shine that does not diminish over time.

Location

England

The jet found at Whitby, in England, is the "Jet Rock" unit of the Mulgrave Shale Member, which is part of the Whitby Mudstone Formation. This jet deposit was formed approximately 181 million years ago, during the Toarcian age of the Early Jurassic epoch. Whitby Jet is the fossilized wood from species similar to the extant Chilean pine (Araucaria araucana). The deposit extends throughout North York Moors National Park.

Jet has also been found in Kimmeridge shale seams in Dorset.

France

Jet was mined from a number of areas of France including Montjardin and Roquevaire. Raw jet was also imported from Spain. In the 18th century there was a jet working industry based around Sainte-Colombe-sur-l'Hers and La Bastide-sur-l'Hers but this declined with the start of the 19th. An 1871 plan to import raw French jet into Whitby was unsuccessful due to its poor quality.

Spain

The jet found in Asturias, the biggest deposit in northern Spain, is of Late Jurassic (Kimmeridgian) age, about 155 million years old. Asturian jet is a perhydrous coal that suffered an anomalous coalification process and presents great material stability over long periods of time. At the end of the Middle Ages, the trade of religious objects and amulets made of jet reached great development in Santiago de Compostela, with sales to pilgrims traveling the Camino de Santiago. However, the deposits were in Asturias, where simple objects such as beads and rosary beads were also made. Santiago de Compostela was the main sales point and the location of the workshops that produced artistic objects. Jet has also been extracted in the area of Utrillas, Gargallo, and Montalbán in the province of Teruel, although it is of lower quality than that from Asturias.

United States

Native American Navajo and Pueblo tribes of New Mexico were using regionally mined jet for jewelry and the ornamentation of weapons when early Spanish explorers reached the area in the 1500s. Today these jet deposits are known as Acoma jet, for the Acoma Pueblo. Enormous coal deposits characterize the San Juan Basin of New Mexico and this geology is closely related to jet deposits mined in the Henry Mountains of Utah and the Front Range of El Paso County, Colorado.

Other locations

Jet is also commercialized in Poland and near Erzurum in Turkey, where it is known as oltu stone and is used to make prayer beads.

History

The earliest known worked jet object is a 10,000 BC model of a botfly larva, from Baden-Württemberg, Germany, found among the Venuses of Petersfels.

Because of its luster and ease of carving, jet was already used to make jewelry in prehistoric times, for example in the late Hallstatt period and the early La Tène period in southern Germany. The small stylized female figures of the Magdalenian period are also jewelry pendants. These Venus statuettes were made from jet in large numbers at the Petersfels. It was not until 1990 that almost identical jet figurines were discovered at the same dated site of Monruz on Lake Neuchâtel in Switzerland. Jet pendants (Lady of Monteloup), later also with amber sliders, are known. The Romans made jewelry, spinning tools (spindle whorls and distaffs), and amulets from jet. From the Middle Ages onwards, mourning jewelry and rosaries were made from it in Europe.

Jet has been used in Britain since the Neolithic period It continued in use in Britain through the Bronze Age where it was used for necklace beads. Jet necklaces following the plate and spacer design may have been based on Gold lunula. During the Iron Age jet went out of fashion until the early 3rd century AD in Roman Britain. The end of Roman Britain marked the end of jet's ancient popularity.

Early archaeologists (particularly Victorian) often failed to distinguish between jet and other jet-like materials In particular in southern Britain the material described as jet was often Kimmeridge Shale. and some artifacts use more than one jet-like material. For example, the Pen y Bonc necklace combines two or three jet pieces with other dark material.

Pliny the Elder attributed healing properties to jet. It was said to protect against the evil eye, ward off snakes, cure hysteria and toothaches, overcome epilepsy, and help determine virginity. In gemstone therapy, jet is considered a mourning stone.

At the end of the 19th century, during the heyday of jet fashion, when deposits became rarer, ebonite, a hard rubber, was used as a substitute for jet. The main deposit at that time was on the north coast of England near the port town of Whitby. There are other deposits in the Spanish region of Asturias (Villaviciosa), southern France, Austria (Gams near Hieflau and in the Reichraminger Hintergebirge - Am Sandl) and in Württemberg. Geologically, the deposit near Whitby is assigned to the Liassic (Black Jurassic). Jet is still found today on the nearby cliffs. However, not all deposits can be assigned to the Jurassic (see, for example, Bechtel et al.).

The Spanish finds went largely to the guild of jet carvers of Santiago de Compostela (Cofradía de los azabacheros de Santiago), who, in addition to jewelry and devotional items, made pilgrimage signs and souvenirs in the form of scallop shells or the so-called Santiago fica, which was considered a defense against the evil eye. The concentration of jet carvers around a square at the Cathedral of Santiago de Compostela is preserved to this day in its name Plaza de Azabache or Azabachería (Jet Square).

Uses

It is used in jewelry to create beautiful pieces, such as pendants, cameos, necklaces, and rings, along with materials such as finewoods, precious or semiprecious stones, and precious metals. Larger pieces are also used for carving. For thousands of years, jet has been attributed with a protective character against all evil. It is considered the talisman of the Camino de Santiago, the protector of the pilgrim, the magic stone, the Jacobean emblem.

Roman use

Whitby jet was a popular material for jewellery in Roman Britain from the 3rd century onward. There is no evidence for Roman jet working in Whitby itself, rather it was transferred to Eboracum (modern York) where considerable evidence for jet production has been found. The collection of jet at this time was based on beachcombing rather than quarrying. It was used in rings, hair pins, beads, bracelets, bangles, necklaces, and pendants, many of which can be seen in the Yorkshire Museum. Jet rings tended to follow the styles of existing metal rings although there were exceptions. Jet pendants were carved cameo style with Medusa head being a popular theme.

Stylistic similarities with jet items found in the Rhineland, and lack of any evidence for local manufacture, suggest that Eboracum-produced items were exported to that area. One item that has been found around the Rhine but not in Britain are jet bracelets that feature grooves with gold inserts.

The Roman period saw its use as a magical material, frequently used in amulets and pendants because of its supposed protective qualities and ability to deflect the gaze of the evil eye. Pliny the Elder suggests that "the kindling of jet drives off snakes and relieves suffocation of the uterus. Its fumes detect attempts to simulate a disabling illness or a state of virginity." It has been referenced by other ancient writers including Solinus and Galen.

Viking use

Vikings made some use of jet including rings and miniature sculptures of animals with snakes being a prominent theme. The source of the jet has not been confirmed although Whitby is the most likely possibility.

Medieval

Medieval jet use appears to have been largely limited to religious items such as crosses and Rosary beads. During the period there was a belief that water drunk from jet bowls could help with labour. A jet bowl held in the Museum of London may have been designed to allow for this.

Jet became a valued costume accessory in the 16th century. Mary, Queen of Scots, owned jet buttons and clothes embroidered with jet beads. Elizabeth I bought 1000 "black jet bugle drops" to embroider headdresses in 1587. Anne of Denmark ordered a gown of "double burret" silk in June 1597 loaded with jet passementerie and 360 jet buttons. The gown was too heavy to wear and she ordered it to be remade with less jet.

Victorian use

Jet as a gemstone became fashionable during the reign of Queen Victoria. It originally became fashionable in the 1850s after the queen wore a necklace of it as part of mourning dress for Princess Victoria of Saxe-Coburg and Gotha. Later the Queen wore Whitby jet as part of her mourning dress while mourning the death of Prince Albert.

In some jewellery designs of the period jet was combined with cut steel.

Jet use was at its highest in the early 1870s and from there it declined. From above 1000 workers in the trade Whitby was down to 300 by 1884. While jet substitutes may have had an impact this appears to have been in a large part due to changes in fashion with Art Nouveau making little use of black jewellery. As the numbers fell the remaining manufactures tended to stick with existing styles rather than attempting to adapt to new fashions resulting in demand falling further. Making tourist trinkets kept a few jewellers in work, but by the end of World War II only three remained, and the industry died out completely with their deaths.

20th century

In Whitby the Victorian tradition continued up until the aftermath of World War II. Jet jewellery (both vintage and new) was then to remain out of fashion until the late '70s. In the '80s there was a fashion for jet beads and antique jet jewellery started to rise in value. New jewellers took up the production of jet jewellery.

Artificial jet gemstone

Containing the word "jet", "Jet of Paris" or "French jet" jewelry is a product made of glass: "black glass obtained with a mixture of copper oxide, cobalt and iron". Genuine jet leaves a dark brown, almost black trace on a rough surface, while glass and other imitations leave a white trace. There is also a more modern fake jet available in stores, which does not have the properties of quality jet.

Jet gemstone substitutes

Glass was used as a jet substitute during the peak of jet's popularity. When it was used in this way it was known as French jet or Vauxhall glass. Ebonite was also used as a jet substitute and initially looks very similar to jet, but it fades over time. In some cases jet offcuts were mixed with glue and molded into jewelry.

Anthracite (hard coal) is superficially similar to fine jet, and has been used to imitate it. This imitation is not always easy to distinguish from real jet.

Some museums have produced reproductions of jet artefacts in epoxy resin.

Imitations

Because jet resembles the rare onyx, it is sometimes used as a base material for imitations. However, colored agate and schorl are increasingly being used as imitation materials for onyx and also for jet, as the latter's low Mohs hardness of 2.5 to 4 makes it very susceptible to damage (especially scratches).

Jet can also be confused with and imitated with anthracite coal, asphalt, canned or sapropel coal as well as colored glass, hard rubber and plastic. There are other materials that, because they have a more or less similar appearance and color to jet, are confused with it and are sometimes sold under that name without actually being so. These include:

Lignite. Although it has a composition similar to real jet, its properties and characteristics are very different. Jet has a carbon percentage around 85% and was formed in theJurassicperiod, while lignite has a carbon percentage between 40-70% and was formed in theCretaceousperiod. When to shape it to make objects or ornaments, black lignite fissures and cracks easily, for this reason there is very old news from the jet makers' guild of Santiago de Compostela in which these professionals rejected lignite with which it was impossible to make important pieces, since it often happened that the work was ruined after many hours of effort due to material failures, however it is possible to make small pieces of jewelry confusion with lignite, so the sometimes interested, has survived for centuries. 

Obsidian

Black Jade

Ebonite. From the 1850s onwards, this synthetic material was used as an imitation of jet, as it could be shaped by heating and was much easier to work with. 

Authenticating jet gemstone

Unlike black glass, which is cool to the touch, jet is not cool, due to its lower thermal conductivity. When rubbed against unglazed porcelain, true jet will leave a brown streak, although bog oak, vulcanite, and lignite will do the same.

When non destructive testing is required, X-ray fluorescence spectroscopy, combined with visual inspection (including under high magnification) and X-ray imaging, is generally effective, although it can be difficult to differentiate jet from lignite.

Real jet, when placed in a flame, burns like coal and gives off a coal-like smell and produces soot. No other black "gemstone" behaves like this.

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