Jet is* really a kind of fossilized coal, or a brown lignite, which has formed over a period of many thousands of years. It is not as hard as diamond so it can’t be used for diamond wedding bands. It is therefore essentially a combination of hydrogen and carbon. It usually occurs in alum shales, often as compact masses, the outer surface being marked with longitudinal striae. The transverse fracture, which is conchoidal, has a resinous luster, and a good surface polish may be obtained, despite the low hardness, which is only 31/^ to 4. Specific gravity is about 1.35, but this may range between 1.10 and 1.40; the refractive index is 1.64 to 1.68.
A bad conductor of heat, jet feels warm to the touch. Friction develops electricity; heat will cause it to burn, giving off dense, pungent fumes. These fumes were once thought to have valuable medicinal properties, but this was no doubt based on superstition. Jet was used in medicine many centuries ago as well as a means of incantation in the art of magic. It has also been used in the staining of pottery.