Jet

Jet (Fr. jais, Ger. Gagat), a substance which seems to be a peculiar kind of lignite or anthracite; often cut and polished for ornaments. The word “jet” probably comes, through O. Fr. jaiet, from the classical gagates, a word which was derived, according to Pliny, from Gagas, in Lycia, where jet, or a similar substance, was originally found. Jet was used in Britain in prehistoric times; many round barrows of the Bronze age have yielded jet beads, buttons, rings, armlets and other ornaments. The abundance of jet in Britain is alluded to by Caius Julius Solinus (fl. 3rd century) and jet ornaments are found with Roman relics in Britain. Probably the supply was obtained from the coast of Yorkshire, especially near Whitby, where nodules of jet were formerly picked up on the shore. Caedmon refers to this jet, and at a later date it was used for rosary beads by the monks of Whitby Abbey.

The Whitby jet occurs in irregular masses, often of lenticular shape, embedded in hard shales known as jet-rock. The jet-rock series belongs to that division of the Upper Lias which is termed the zone of Ammonites serpentinus. Microscopic examination of jet occasionally reveals the structure of coniferous wood, which A. C. Seward has shown to be araucarian. Probably masses of wood were brought down by a river, and drifted out to sea, where becoming water-logged they sank, and became gradually buried in a deposit of fine mud, which eventually hardened into shale. Under pressure, perhaps assisted by heat, and with exclusion of air, the wood suffered a peculiar kind of decomposition, probably modified by the presence of salt water, as suggested by Percy E. Spielmann. Scales of fish and other fossils of the jet-rock are frequently impregnated with bituminous products, which may replace the original tissues. Drops of liquid bitumen occur in the cavities of some fossils, whilst inflammable gas is not uncommon in the jet-workings, and petroleum may be detected by its smell. Iron pyrites is often associated with the jet.

Formerly sufficient jet was found in loose pieces on the shore, set free by the disintegration of the cliffs, or washed up from a submarine source. When this supply became insufficient, the rock was attacked by the jet-workers; ultimately the workings took the form of true mines, levels being driven into the shales not only at their outcrop in the cliffs but in some of the inland dales of the Yorkshire moorlands, such as Eskdale. The best jet has a uniform black colour, and is hard, compact and homogeneous in texture, breaking with a conchoidal fracture. It must be tough enough to be readily carved or turned on the lathe, and sufficiently compact in texture to receive a high polish. The final polish was formerly given by means of rouge, which produces a beautiful velvety surface, but rotten-stone and lampblack are often employed instead. The softer kinds, not capable of being freely worked, are known as bastard jet. A soft jet is obtained from the estuarine series of the Lower Oolites of Yorkshire.

Much jet is imported from Spain, but it is generally less hard and lustrous than true Whitby jet. In Spain the chief locality is Villaviciosa, in the province of Asturias. France furnishes jet, especially in the department of the Aude. Much jet, too, occurs in the Lias of Württemberg, and works have been established for its utilization. In the United States jet is known at many localities but is not systematically worked. Pennsylvanian anthracite, however, has been occasionally employed as a substitute. In like manner Scotch cannel coal has been sometimes used at Whitby. Imitations of jet, or substitutes for it, are furnished by vulcanite, glass, black obsidian and black onyx, or stained chalcedony. Jet is sometimes improperly termed black amber, because like amber, though in less degree, it becomes electric by friction.

See P. E. Spielmann, “On the Origin of Jet,” Chemical News (Dec. 14, 1906); C. Fox-Strangways, “The Jurassic Rocks of Britain, Vol. I. Yorkshire,” Mem. Geol. Surv. (1892); J. A. Bower, “Whitby Jet and its Manufacture,” Journ. Soc. Arts (1874, vol. xxii. p. 80).