Ch. 6:Other Gems

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	JET.	353

	amber of fifteen pounds weight is preserved in the cabinet at Berlin. The inhabitants of Colberg, in 1576, presented to the Emperor Rudolph II. a specimen weighing eleven pounds. JET. This mineral occurs massive ; has a conchoidal fracture ; is opaque; has a shining lustre; and is of a jet, or pitch-black color. It is pretty soft, and yields to the knife; its hardness is 1.0 to 2.5; specific gravity, 1.29 to 1.35; it burns with a greenish flame, and emits a strong bituminous smell. In trade it is also called black amber, or pitch coal. It is found in the brown-coal formation, the plastic clay, and the lias, with lignite and amber, in England, France, Silesia, Hesse, Italy, Spain, and Prussia. Jet bears a high polish, and is wrought into necklaces, ear-rings, crosses, rosaries, snuff-boxes, buttons, bracelets, and particularly mourning jewelry. It is at first generally assorted to select the best pieces, most suitable for working; such as are free from iron pyrites, lignite, and have no cracks or fissures. It is then turned on a lathe, and likewise on horizontal sandstone wheels, which run unequally on their periphery, by which the various specimens may be cut and polished at the same time. During the operation the jet must be moistened with water, else it may crack from being overheated. It is polished with rotten-stone or crocus martis and oil, on linen or buckskin; and lastly by the palm of the hand. The manufacturing of jet ornaments was formerly a considerable branch of industry in France, where, in 1786, the department de l'Aube occupied twelve hundred workmen ; but at the present time it is not worn, and the black enamel is substituted for it. Jet is a species of bituminous coal, which has several

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