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Jet is a type of lignite, a precursor to coal, and is a gemstone. Jet is not a mineral, but rather a mineraloid. It has an organic origin, being derived from wood that has decayed under extreme pressure. The English noun “jet” derives from the French word for the same material, jaiet (modern French jais), ultimately referring to the ancient town of Gagae. Jet is either black or dark brown, but may contain pyrite inclusions, which are of brassy colour and metallic lustre. The adjective “jet-black”, meaning as dark a black as possible, derives from this material.
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. Whitby Jet is the fossilised wood from species similar to the extant Chile pine or Monkey Puzzle tree (Araucaria araucana).