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A sample of jet

Jet is a geological material not considered a true mineral, but rather a mineraloid derived from decaying wood under extreme pressure, and thus organic. The English word-name "jet" derives from the French word for the same material: jaiet. Jet is either black or dark brown, but may contain pyrite inclusions, which are of brassy color and metallic lustre. The adjective jet-black is better-known perhaps than the substance from which the descriptive phrase derives.^[1]

Hallstatt culture bracelets made from jet and bronze, unearthed at Magdalenenberg

Jet is a product of high pressure decomposition of wood from millions of years ago, commonly of the Araucariaceae family. It 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. Jet is easily polished and is used in manufacturing jewellery, according to the Whitby Museum, dating from 10,000 BC in parts of contemporary Germany.

Mourning jewellery: Jet Brooch, 19th century.

Jet as a gem material was highly popular during the reign of Queen Victoria, during which the Queen wore Whitby jet as part of her mourning dress. Jet was popular for mourning jewellery in the 19th century because of its sombre color 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 1920s, or 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. Powdered jet added to water or wine was believed to have medicinal powers.

Although now much less popular, authentic jet jewels are valued by collectors. Anthracite (hard coal) and vulcanite are similar materials that have been used to imitate fine jet: these imitations are not always easy to distinguish from the real thing. Unlike black glass which is cool to the touch, jet is not, due to its lesser thermal conductivity. Also, the structure of jet (which is remarkably like the wood that it is derived from) can be seen under 120X or greater microscopes.

• Whitby Jet
• Jet, the fossilized wood, the Asturian Jet in the Jurassic.

1. ^ http://www.word-detective.com/120505.html#jet