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Jet (lignite)

Topic: Earth

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Sample of unworked jet (about 15mm long)
Three views of a prehistoric pendant in lignite/jet; Magdalenian culture (17,000–10,000 BCE), from the Marsoulas cave, Marsoulas, Haute-Garonne, France (Muséum de Toulouse)
Hallstatt culture bracelets made from jet and bronze, unearthed at the princely tomb of Magdalenenberg
Mourning jewellery: jet brooch, 19th century

Jet is a type of lignite,[1] a precursor to coal, and is a gemstone. Jet is not a mineral, but rather a mineraloid.[2] It has an organic origin, being derived from wood that has decayed under extreme pressure.

The English noun "jet" derives from the French word (modern French jais) for the same material: jaiet.[3] Jet is either black or dark brown, but may contain pyrite inclusions,[4] which are of brassy colour and metallic lustre. The adjective "jet-black", meaning as dark a black as possible, derives from this material.

Origin

Jet is a product of high-pressure 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 is the result of carbon compression and fresh water.

The jet found at Whitby, in England, is of early Jurassic (Toarcian) age, approximately 182 million years old.[5][6] Whitby Jet is the fossilized wood from species similar to the extant Chile pine or Monkey Puzzle tree (Araucaria araucana).[7]

Other locations

Jet is also found in Poland and Santiago de Compostela in northern Spain[8] and near Erzurum in Turkey, where it is used to make prayer beads.[9]

History

Jet has been used in Britain since the Neolithic period,[10] 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.[11][10] It continued in use in Britain through the Bronze Age where it was used for necklace beads.[10] 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[10], 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.[10]

Roman use

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,[10] many of which are visible in the Yorkshire Museum. There is no evidence for Roman jet working in Whitby itself,[10] rather it was transferred to Eboracum (modern York) where considerable evidence for jet production has been found.[12] The collection of jet at this time was based on beachcombing rather than quarrying.[10]

Jet cameo depicting a Medusa in the Yorkshire Museum

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.[13] 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."[14] It has been referenced by other ancient writers including Solinus[15] and Galen.

Jet objects were exported from Eboracum all over Roman Britain and into Europe.[10]

Around the Rhine some jet bracelets from the period have been found that feature grooves with gold inserts.[16]

Victorian use

A large piece of jet from Whitby

Jet as a gemstone was fashionable during the reign of Queen Victoria, during which the Queen wore Whitby jet as part of her mourning dress, mourning the death of Prince Albert.[7] Jet was associated with mourning jewellery in the 19th century because of its sombre colour and modest appearance,[17] and it has been traditionally fashioned into rosaries for monks.

In some jewellery designs of the period jet was combined with cut steel.[18]

20th century

In the United States, long necklaces of jet beads were popular during the Roaring Twenties, when women and young flappers would wear multiple 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.

Properties

Jet has also been known as black amber, as it may induce an electric charge like that of amber when rubbed.

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.[19]

Authenticating jet

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.[20][21] When it was used in this way it was known as French jet or Vauxhall glass.[20][21] Ebonite was also used as a jet substitute and initially looks very similar to jet, but it fades over time.[22] In some cases jet offcuts were mixed with glue and molded into jewellery.[22]

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.

See also

References

  1. ↑ Neuendorf, K.K.E.; Mehl, Jr., J.P.; Jackson, J.A. (editors) (2005). Glossary of Geology (5th edition). Alexandria, Virginia: American Geological Institute. pp. 344. 
  2. ↑ Holmes, Ralph J. and Crowningshield, Robert (1983), "Gemology", in Fyfe, Keith (editor), The Encyclopedia of Mineralogy, Springer, pp. 168–187, ISBN 978-0-87933-184-9, archived from the original on 2015-06-11, https://link.springer.com/referenceworkentry/10.1007%2F0-387-30720-6_51 
  3. ↑ Oxford English Dictionary (2nd edition) 1989, Oxford, Oxford University Press
  4. ↑ Pye, K. (1985) Electron microscope analysis of zoned dolomite rhombs in the Jet Rock Formation (Lower Toarcian) of the Whitby area, U.K., Geological Magazine, volume 122, pp. 279–286, Cambridge University Press, doi:10.1017/S0016756800031496
  5. ↑ Cope, J. C. W. (2006) Jurassic: the returning seas - plate 26 and page 339 of Brenchley, P. J. and Rawson P. F. (editors) (2006) The Geology of England and Wales, 2nd edition, London, The Geological Society
  6. ↑ "Jurassic timescale" . Retrieved 2010-06-23.
  7. ↑ 7.0 7.1 Oliver, N., 2012, A History of Ancient Britain, Phoenix Paperback, ISBN:978-0753828861
  8. ↑ "Some was Polish, but most came from near Santiago de Compostela in northern Spain" Finlay, Victoria. Jewels: A Secret History (Kindle Location 1035). Random House Publishing Group. Kindle Edition.
  9. ↑ "While there is no source of jet anywhere near southwestern Turkey, it can be found in western Anatolia near Erzurum, where there are about six hundred family-run mines in the mountains. They call it oltu-tasi and it is the material from which Muslim prayer-beads are made. Finlay, Victoria. Jewels: A Secret History (Kindle Locations 1054-1056). Random House Publishing Group. Kindle Edition.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 Allason-Jones, L. 1996, Roman Jet in the Yorkshire Museum, York: Yorkshire Museum. pp. 8–11 ISBN:978-0905807171
  11. ↑ "Venus figures from Petersfels". Archived from the original on 29 September 2016. https://web.archive.org/web/20160929213049/http://donsmaps.com/petersfels.html. 
  12. ↑ Ottaway, P., 2004, Roman York, Tempus: Stroud ISBN:978-0752429168
  13. ↑ Henig, M., 1984, Religion in Roman Britain, London, BT Batsford LTD ISBN:978-0312670597
  14. ↑ Pliny the Elder. Natural History (trans. Bostock, J., Riley, H. T.). London: Taylor and Francis. 1855. Chapter 36
  15. ↑ Caius Julius Solinus (2013). "DE MIRABILIBUS MUNDI CAPITULA VII - XXIV". Archived from the original on 2013-02-06. https://web.archive.org/web/20130206160554/http://thelatinlibrary.com/solinus2.html. Retrieved 2013-10-31. 
  16. ↑ Johns, Catherine (1996). The Jewellery of Roman Britain Celtic and classical Traditions. Routledge. pp. 120–121. ISBN 9780415516129. 
  17. ↑ Phillips, Clare (1996). Jewelry from Antiquity to the Present. Thames and Hudson. pp. 148–150. ISBN 0500202877. 
  18. ↑ Clifford, Anne (1971). Cut-Steel and Berlin Iron Jewellery. Adams & Dart. pp. 21–22. ISBN 9780239000699. 
  19. ↑ Richard T. Liddicoat, Jr. Handbook of Gem Identification 1989 GIA press, 12 ed. pg 192
  20. ↑ 20.0 20.1 "Jet necklace and black silk tie". Pitt Rivers Museum. 2011. Archived from the original on 16 February 2016. https://web.archive.org/web/20160216001040/http://web.prm.ox.ac.uk/bodyarts/index.php/body-arts-and-lifecycles/death/118-jet-necklace-and-black-silk-tie.html. 
  21. ↑ 21.0 21.1 "Tiara". Victoria and Albert Museum. Archived from the original on 15 February 2016. https://web.archive.org/web/20160215214703/http://collections.vam.ac.uk/item/O120604/tiara-unknown/. 
  22. ↑ 22.0 22.1 Phillips, Clare (1996). Jewelry from Antiquity to the Present. Thames and Hudson. p. 150. ISBN 0500202877. 

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