Jet
3 of the 7 encyclopedias on this shelf carry an entry for Jet. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a peculiar form of pitch coal. It is a solid, black, dry, inflammable substance. It occurs as a mineral in veins of lignite coal. When first mined jet may be carved and turned readily into toys, buttons, and articles of ornament. It takes a high polish. The finest jet in the world is found in a vein twenty feet thick at Whitby, Yorkshire. The monastery of Whitby did a thriving business in jet rosaries and crosses when that shrine was a resort of pilgrims. Other fine veins are found in the provinces of Aude, France, and Asturias, Spain. The original jet quarry was in Asia Minor . The Greek name, of which jet is a corruption, was gagates. Jet was prized highly by the ancients and is still the appropriate jewelry of mourning, though cheap imitations in glass render genuine jet ornaments less desirable. Its deep lustrous black has made jet a familiar standard of comparison. Milton speaks of the "pansy, freaked with jet," and Scott, of the black-cock's "jetty wing." See Asphalt
An Encyclopaedia of Occultism (1920)
Its virtues are thus described by Pliny, according to the version of Holland : In burning, the perfume thereof. chaseth away serpents, and bringeth women again that lie in a traunce by the suffocation or rising of the mother; the said smoke discovereth the falling sicknesse and bewraieth whether a young damsel be a maiden or no; the same being boiled in wine helpeth the toothache, and tempered with wax cureth the swelling glandules named the king's evil. They say that the magicians use this jeat stone much in their sorceries, which they practice by the means of red hot axes, which they call axinomancia, for they affirm that being cast thereupon it will burne and consume, if that ewe desire and wish shall happen accordingly." Jet is known in Prussia as black amber.
Collier's New Encyclopedia (1921)
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, afterward corrupted into gagat, the modern German name, and jet. As a material for mourning ornaments jet is admirably adapted, and for that purpose is largely used.