Sample of unworked jet, about 15 mm long
|Color||Black, occasionally brown|
|Mohs scale hardness||2.5-4.0|
|Refractive index||1.640 to 1.680|
|Common impurities||Iron, sulfur|
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.
The English noun jet derives from the French word for the same material, jaiet (modern French jais), ultimately referring to the ancient town of Gagae. 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 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.
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 Chile pine or Monkey Puzzle tree (Araucaria araucana).
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.
Native American Navajo and Pueblo tribes of New Mexico were using regionally mined jet for jewellery 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.
Jet has been used in Britain since the Neolithic period, but the earliest known object is a 10,000 BC model of a botfly larva, from Baden-Württemberg, Germany, found among the Venuses of Petersfels. It continued in use in Britain through the Bronze Age where it was used for necklace beads. 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, despite sporadic use in the Anglo-Saxon and Viking periods and the later Medieval period. Jet regained popularity with a massive resurgence during the Victorian era.
Whitby jet was a popular material for jewellery in Roman Britain from the 3rd century onward. It was used in rings, hair pins, beads, bracelets, bangles, necklaces, and pendants, many of which are visible in the Yorkshire Museum. 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.
In the Roman period it saw 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.
Around the Rhine some jet bracelets from the period have been found that feature grooves with gold inserts.
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.
Jet continued to be used in jewellery even after the decline of mourning dress in the 1890s.
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In the United States, long necklaces of jet beads were popular during the Roaring Twenties, when women and young flappers would wear strands of jet beads stretching from the neckline to the waistline. In these necklaces, the jet was strung using heavy cotton thread; small knots were made on either side of each bead to keep the beads spaced evenly, much in the same way that fine pearl necklaces are made.
Jet is very easy to carve, but it is difficult to create fine details without breaking so it takes an experienced lapidary to execute more elaborate carvings.
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.
Although now much less popular than in the past, authentic jet jewels are valued by collectors.
Unlike black glass, which is cool to the touch, jet is not cool, due to its lower thermal conductivity. 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. When rubbed against unglazed porcelain, true jet will leave a chocolate brown streak.
The microstructure of jet, which strongly resembles the original wood, can be seen under 120× or greater magnification.
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.