The lustrous, opaque, black material known as jet has been and used by mankind for centuries. Jet ornaments have been found in Paleolithic cave deposits in Switzerland and France and among the archeological treasures of the Pueblo Indians of North America. During the days of the Roman occupation of Britain and up to the present time, it has had a varying degree of popularity, finding its widest use as mourning jewelry in the middle of the nineteenth century. The name originated from a Greek word for an unspecified place in Asia Minor that was then known as Lycia, progressed through the Latin form "gagates" and came to us eventually through the old French "jaiet".
Jet is a variety of brown coal known as lignite (largely carbon, with hydrogen and oxygen), which is from the Latin word Lignum, meaning wood. It formed as a result of the compaction of driftwood that sank to the sea bottom and became embedded in fine grained mud, which was later transformed into a hard shale known as "jet rock". Thus, jet is organic in original. It is in this shale formation, on and near the coast in the vicinity of Whitby, Yorkshire, England, that the finest jet has always been found and worked. It occurs in lumps of variable size, often retaining the shape of the branches and twigs of the trees from which it evolved. There are also other sources of jet. Spanish jet (found in Aragon, Galicia and Austria) is harder and more brittle than the English product. That from Wurtemberg, Germany, is considered inferior. Material from El Paso County, Colorado, takes a fine polish, and Utah jet (Wayne Country) is generally an inferior quality with many cracks. Deposits also occur at Pictou, Nova Scotia, Canada but the hardness of the material produced is inferior. The province of Languedoc, France, is another long-known source.
Jet is amorphous (and therefore isotropic), has a dull to greasy luster on rough surfaces and nearly vitreous luster on polished surfaces, a hardness of 3 to 4, a conchoidal fracture, an R.I. 1.64 to 1.68, an S.G. ranging from 1.10 to 1.40, a brown streak, and a degree of toughness (in good quality) that permits it to be carved or worked easily on a lathe. Acids may dull the surface, and the heat of the blowpipe or jeweler's torch will cause jet to burn with a sooty flame and to emit an unpleasant odor.
Other black materials that resemble jet include dyed chalcedony, black tourmaline (schorl), garnet (melanite) and obsidian; all of these have a much higher specific gravity and are much harder and colder to the touch. Anthracite coal is similar in appearance, but it is lighter, less compact, and more brownish than genuine jet. Glass is readily detected by its vitreous luster on fracture surfaces and by its greater hardness and higher S.G. Bakelite has a harder luster and emits a carbolic-acid odor when heated. Galalith also has this so called harder luster, but gives off the odor of formaldehyde upon the application of heat. Vulcanite, strongly vulcanized rubber, emits the unmistakable odor of rubber when heated; moreover; it is usually molded, thus lacking the sharp edges that can be cut on jet.
Jet has been affected in popularity by the dictates of fashion. As mentioned above, it was in vogue for mourning jewelry in the nineteenth century, particularly during the rule of Queen Victoria, but mourning artier is no longer in vogue. Beads (often faceted), cross-shaped pendants, bracelets, brooches, rosaries, cane handles, snuff boxes, ink stands and all manner of jewelry, ornamental and utilitarian objects have been fashioned from jet. Purity of the black color, freedom from inclusions of other minerals (especially pyrite and sulphur), and absence of fine cracks increase its value, which is never particularly high. The more compact, dense and harder types are the most desirable. The polish should be bright; a dull, polish is considered undesirable.