Jet (lignite)
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Jet is a geological material and is considered to be a minor gemstone. Jet is not considered a true mineral, but rather a mineraloid as it has an organic origin, being derived from decaying wood under extreme pressure.
The English noun "jet" derives from the French word for the same material: jaiet.[1] Jet is either black or dark brown, but may contain pyrite inclusions,[2] which are of brassy colour and metallic lustre. The adjective "jet-black", meaning as dark a black as possible, derives from this material.
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[edit] 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 the carbon compression and salt water; soft jet is the result of the carbon compression and fresh water.
The jet found at Whitby, England is of early Jurassic (Toarcian) age, approximately 182 million years old.[3][4]
[edit] History
Jet is easily polished and is used in manufacturing jewellery, according to the Whitby Museum, dating from 10,000 BC in parts of Germany. The oldest jet jewellery was found in Asturias, Spain, dating from 17,000 BC. Whitby jet was a favourite material for ornamental jewelllery during the Roman period, when it was described by Solinus. Hair-pins, rings, spindles, bracelets, and necklaces were produced in great variety, much of it at Eburacum (modern York).[5] 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.[6] Furthermore, Pliny the Elder suggests that "the kindling of jet drives off snakes and relieves suffocation of the uterus. Its fumes detect attempts to stimulate a disabling illness or a state of virginity.” [7]
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. Jet was associated with mourning jewellery in the 19th century because of its sombre colour and modest appearance, and it has been traditionally fashioned into rosaries for monks. In the United States, long necklaces of jet beads were very 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. Jet has also been known as black amber, as it may induce an electric charge like that of amber when rubbed.
[edit] Properties
Jet is very easy to carve but it is difficult to create detail without breaking so it takes some time for learning and executing more elaborate carving.
Jet has a Mohs hardness ranging between 2.5 to 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 odor similar to coal.[8]
[edit] 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 lesser thermal conductivity.
Anthracite (hard coal) and vulcanite (hardened rubber) are superficially similar to fine jet, and have been used to imitate it. These imitations are 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 is remarkably like that of the wood it is derived from) can be seen under 120x or greater magnification.
[edit] References
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- ^ Oxford English Dictionary (2nd edition) 1989, Oxford, Oxford University Press
- ^ 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
- ^ 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
- ^ Jurassic timescale Retrieved 2010-06-23
- ^ Richmond, I. A. (1955). Roman Britain. The Pelican History of England. Harmondsworth, Middlese: Penguin Books. p. 161.
- ^ Henig, M., 1984. Religion in Roman Britain. London: BT Batsford LTD
- ^ Pliny the Elder. Natural History (trans. Bostock, J., Riley, H. T.). London: Taylor and Francis. 1855. Chapter 36
- ^ Richard T. Liddicoat, Jr. Handbook of Gem Identification 1989 GIA press, 12 ed. pg 192
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