Jet, a dense variety of lignite passing by degrees of quality into bituminous fossil wood, sometimes perfectly black, capable of being easily cut and carved, and of receiving a very beautiful polish. It takes its name from Gagas or Gages, a place in Asia Minor, where, according to Pliny, the substance was obtained, whence in his time it was called gagates, afterwards corrupted into gagat, the modern German name, and jet. Jet is only a peculiar form of lignite, impregnated with bituminous matter, and containing about 37½ per cent. of volatile matter. It is electrical when rubbed; hence it has been called 'black amber' by the Prussian amber-diggers.
Of substances used for trinkets and personal ornament, apart from metals, jet appears to be one of the most ancient. At numerous places throughout Great Britain necklaces, beads, buttons, and other small objects of jet have been discovered, showing that it had been used in the early bronze period. Probably at that remote time it was obtained from the Yorkshire coast about Whitby, whence the principal supply and the finest quality anywhere obtained continues to come. The jet occurs at Whitby in irregular interbedded patches in the Upper Lias shales, two kinds, hard and soft, being found; but only the hard is of value for ornaments. The industry there gives employment to a large proportion of the population. It is also worked in France in the department of Aude, where it is formed into rosary beads, crosses, and other trinkets. Spain also supplies fine jet, which, like that of the French workings, is found in irregular veins in the lower marls of the Cretaceous series, corresponding with the Sussex gault. The Spanish jet is found at Villaviciosa, in the province of the Asturias, and is principally manufactured at Oviedo. As a material for mourning ornaments jet is admirably adapted, and for that purpose is largely used. Imitations of jet ornaments are made in the hardened india rubber called Vulcanite or Ebonite, and in glass.