Jet (gemstone)

Origin and Properties of Jet

• 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.
• Both hard and soft jet occupy the same area of the Mohs scale, but soft jet is more likely to crack when exposed to changes in temperature.
• Jet is around 75% carbon and 12% oxygen, with sulfur and hydrogen making up most of the balance.
• The exact ratios of other elements found in jet vary with the source.
• 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.
• Jet may induce an electric charge like that of amber when rubbed.

Locations of Jet Deposits

• England:
• Whitby, England, is known for its jet deposit, which was formed approximately 181 million years ago.
• Whitby Jet is fossilized wood from species similar to the extant Chilean pine.
• The jet deposit extends throughout North York Moors National Park.
• Jet has also been found in Kimmeridge shale seams in Dorset.
• France:
• Jet was mined from areas such as Montjardin and Roquevaire.
• Raw jet was imported from Spain.
• In the 18th century, there was a jet working industry based around Sainte-Colombe-sur-lHers and La Bastide-sur-lHers.
• An 1871 plan to import raw French jet into Whitby was unsuccessful due to its poor quality.
• Spain:
• The biggest jet deposit in northern Spain is found in Asturias and is approximately 155 million years old.
• Jet is also found near Santiago de Compostela in northwest Spain.
• United States:
• Native American tribes in New Mexico, such as the Navajo and Pueblo, used regionally mined jet for jewelry and ornamentation.
• Jet deposits can be found in the San Juan Basin of New Mexico and the Henry Mountains of Utah.
• Jet deposits are also found in the Front Range of El Paso County, Colorado.
• Other locations:
• Jet is commercialized in Poland.
• Jet is found near Erzurum in Turkey, where it is known as oltu stone and used to make prayer beads.

History of Jet

• The earliest known worked jet object is a 10,000 BC model of a botfly larva from Germany.
• Jet has been used in Britain since the Neolithic period and continued through the Bronze Age.
• During the Iron Age, jet went out of fashion until the early 3rd century AD in Roman Britain.
• Early archaeologists often failed to distinguish between jet and other jet-like materials, such as Kimmeridge Shale.
• Jet was popular for jewelry in Roman Britain and was used in various forms such as rings, hair pins, beads, and pendants.
• Vikings made use of jet, including rings and miniature sculptures of animals.
• The source of the jet used by Vikings is likely Whitby.
• Snakes were a prominent theme in Viking jet sculptures.

Jet Substitutes

• Glass used as a jet substitute during the peak of jet's popularity, known as French jet or Vauxhall glass.
• Ebonite also used as a jet substitute, but fades over time.
• Jet offcuts mixed with glue and molded into jewelry.
• Anthracite (hard coal) used to imitate jet, difficult to distinguish from real jet.
• Museums producing reproductions of jet artifacts in epoxy resin.

Authenticating Jet

• Jet is not cool to the touch, unlike black glass.
• True jet leaves a brown streak when rubbed against unglazed porcelain.
• X-ray fluorescence spectroscopy combined with visual inspection and X-ray imaging effective for non-destructive testing.
• Real jet burns like coal, giving off a coal-like smell and producing soot.
• No other black gemstone behaves like jet when exposed to flame.

See also:

• Petrified wood.
• Oltu stone.

References:

• Neuendorf, K. K. E. Jr.; Mehl, J. P.; Jackson, J. A., eds. (2005). Glossary of Geology.
• Holmes, Ralph J.; Crowningshield, Robert (1983). Gemology.
• Oxford English Dictionary (2nd edition) 1989.
• Pye, K. (1985). Electron microscope analysis of zoned dolomite rhombs in the Jet Rock Formation.
• Muller, Helen; Muller, Katy (2009). Whitby Jet.

Jet (gemstone) Data Sources