JET, in natural history, a kind of fossil substance, the characters of which are these.—It is a solid, dry, opaque, inflammable body, found in large detached masses of a fine and regular structure, having a grain like that of wood, splitting more easily in an horizontal than in any other direction; very light, and moderately hard; not fusible, but readily inflammable, and burning a long time with a fine greenish white flame.
Jet is a substance concerning which many erroneous opinions have been entertained, and which hath been very little understood, even in Britain, where the finest in the world is produced; having been usually confounded with a thing greatly inferior to it in value, the common cannel-coal; so that many believe there is no other jet than that substance: they are, however, easily distinguished from each other by the following characters.—Jet is always found in detached masses lodged in other strata; but cannel-coal constitutes
whole strata of itself. Jet has the grain of wood, and splits horizontally with much greater ease than in any other direction; cannel-coal has no particular direction, and splits any way with equal ease. Jet is but moderately hard; cannel-coal not less so than many stones: jet, when set on fire, flames a long time; cannel-coal but a little while.—Jet is found in Italy, Germany, and the East Indies; but nowhere in such plenty as in England. It is very common in Yorkshire and the other northern counties, and is found in many of the clay-pits about London. By the ancients it was used in medicine, and celebrated as an emollient and discentient; but the modern practice takes no notice of it.