| Jet | |
|---|---|
 Sample of unworked jet, about 15 mm long | |
| 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 |
| Ultravioletfluorescence | None |
| Common impurities | Iron, sulfur |
| References | [citation needed] |
Jet is a type oflignite,[1] the lowestrank ofcoal, and is agemstone. Unlike many gemstones, jet is not amineral, but is rather amineraloid.[2] It is derived from wood that has changed under extreme pressure.
The English nounjet derives from the French word for the same material,jaiet (modern Frenchjais), ultimately referring to the ancient town ofGagae.[3] Jet is either black or dark brown, but may containpyriteinclusions[4] which are of brassy colour and metalliclustre. 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 familyAraucariaceae.[5] Jet is found in two forms, hard and soft.[5] Hard jet is the result of carbon compression and salt water; soft jet may be the result of carbon compression and fresh water.[5] 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.[6]
Jet is around 75% carbon and 12% oxygen with sulfur and hydrogen making up most of the balance.[7] 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.[7] Jet has aMohshardness ranging between 2.5 and 4 and aspecific gravity of 1.30 to 1.34. Therefractive index of jet is approximately 1.66. The touch of a red-hot needle should cause jet to emit an odour similar to coal.[8]
Jet may induce anelectric charge like that of amber when rubbed.[7]
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 experiencedlapidarist to execute more elaborate carvings.
The jet found atWhitby, in England, is the "Jet Rock"[9] unit of the Mulgrave Shale Member, which is part of theWhitby Mudstone Formation. This jet deposit was formed approximately 181 million years ago, during theToarcian age of theEarly Jurassic epoch.[10][11][12] Whitby Jet is thefossilized wood from species similar to the extant Chilean pine (Araucaria araucana).[13] The deposit extends throughoutNorth York Moors National Park.[14]
Jet has also been found inKimmeridge shale seams in Dorset.[15]
Jet was mined from a number of areas of France includingMontjardin andRoquevaire.[16] Raw jet was also imported from Spain.[16] In the 18th century there was a jet working industry based aroundSainte-Colombe-sur-l'Hers andLa Bastide-sur-l'Hers but this declined with the start of the 19th.[16] An 1871 plan to import raw French jet into Whitby was unsuccessful due to its poor quality.[16]
The jet found inAsturias, the biggest deposit in northern Spain,[17] 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.[18] 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.[19]
Native AmericanNavajo andPueblo tribes ofNew Mexico were using regionally mined jet for jewelry and the ornamentation of weapons when early Spanish explorers reached the area in the 1500s.[20] Today these jet deposits are known as Acoma jet, for theAcoma Pueblo. Enormous coal deposits characterize theSan Juan Basin of New Mexico and this geology is closely related to jet deposits mined in theHenry Mountains ofUtah[21] and theFront Range ofEl Paso County,Colorado.[22]
Jet is also commercialized in Poland[23] and nearErzurum in Turkey, where it is known asoltu stone and is used to makeprayer beads.[24]
The earliest known worked jet object is a 10,000 BC model of abotfly larva, from Baden-Württemberg, Germany, found among theVenuses of Petersfels.[25]
Jet has been used in Britain since theNeolithic period[26] It continued in use in Britain through theBronze Age where it was used for necklace beads.[26] Jet necklaces following the plate and spacer design may have been based onGold lunula.[27] During theIron Age jet went out of fashion until the early 3rd century AD inRoman Britain. Theend of Roman Britain marked the end of jet's ancient popularity.[26]
Early archaeologists (particularly Victorian) often failed to distinguish between jet and other jet-like materials[28] In particular in southern Britain the material described as jet was oftenKimmeridge Shale.[28][29] and some artifacts use more than one jet-like material.[30] For example, thePen y Bonc necklace combines two or three jet pieces with other dark material.[30]
Whitby jet was a popular material for jewellery in Roman Britain from the 3rd century onward. There is no evidence for Roman jet working inWhitby itself,[26] rather it was transferred toEboracum (modernYork) where considerable evidence for jet production has been found.[31] The collection of jet at this time was based onbeachcombing rather thanquarrying.[26] It was used in rings, hair pins, beads, bracelets, bangles, necklaces, and pendants,[26] many of which can be seen in theYorkshire Museum. Jet rings tended to follow the styles of existing metal rings although there were exceptions.[32] Jet pendants were carved cameo style withMedusa head being a popular theme.[33]
Stylistic similarities with jet items found in the Rhineland, and lack of any evidence for local manufacture, suggest thatEboracum-produced items were exported to that area.[34] One item that has been found around the Rhine but not in Britain are jet bracelets that feature grooves with gold inserts.[35]
The Roman period saw its use as amagical material, frequently used in amulets and pendants because of its supposed protective qualities and ability to deflect the gaze of theevil eye.[36]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."[37] It has been referenced by other ancient writers includingSolinus[38] andGalen.
Vikings made some use of jet including rings and miniature sculptures of animals with snakes being a prominent theme.[39] The source of the jet has not been confirmed although Whitby is the most likely possibility.[39]
Medieval jet use appears to have been largely limited to religious items such as crosses andRosary beads.[40] During the period there was a belief that water drunk from jet bowls could help withlabour.[41] A jet bowl held in theMuseum of London may have been designed to allow for this.[42]
Jet became a valued costume accessory in the 16th century.Mary, Queen of Scots, owned jet buttons and clothes embroidered with jet beads.[43]Elizabeth I bought 1000 "black jet bugle drops" to embroider headdresses in 1587.[44]Anne of Denmark ordered a gown of "double burret" silk in June 1597 loaded with jetpassementerie and 360 jet buttons. The gown was too heavy to wear and she ordered it to be remade with less jet.[45]
Jet as a gemstone became fashionable during the reign ofQueen Victoria.[46] It originally became fashionable in the 1850s after the queen wore a necklace of it as part of mourning dress forPrincess Victoria of Saxe-Coburg and Gotha.[46] Later the Queen wore Whitby jet as part of hermourning dress while mourning the death ofPrince Albert.[13][47][48]
In some jewellery designs of the period jet was combined withcut steel.[49]
Jet use was at its highest in the early 1870s and from there it declined.[50] From above 1000 workers in the trade Whitby was down to 300 by 1884.[50] While jet substitutes may have had an impact this appears to have been in a large part due to changes in fashion withArt Nouveau making little use of black jewellery.[50] 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.[50] 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.[50]
In Whitby the Victorian tradition continued up until the aftermath of World War II.[50] Jet jewellery (both vintage and new) was then to remain out of fashion until the late '70s.[51] In the '80s there was a fashion for jet beads and antique jet jewellery started to rise in value.[52] New jewellers took up the production of jet jewellery.[52]
Glass was used as a jet substitute during the peak of jet's popularity.[53][54] When it was used in this way it was known asFrench jet or Vauxhall glass.[53][54]Ebonite was also used as a jet substitute and initially looks very similar to jet, but it fades over time.[55] In some cases jet offcuts were mixed with glue and molded into jewelry.[55]
Anthracite (hardcoal) 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 inepoxy resin.[56]
Unlike blackglass, which is cool to the touch, jet is not cool, due to its lowerthermal conductivity. When rubbed against unglazed porcelain, true jet will leave a brownstreak, althoughbog oak,vulcanite, andlignite will do the same.[57]
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.[30]
Real jet, when placed in a flame, burns like coal and gives off a coal-like smell and producessoot. No other black "gemstone" behaves like this.
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