Jet is fossilized coal. Famous for it's lustrous, pure black color, jet was used in Victorian-era mourning jewelry (when Queen Victoria's husband, Prince Albert died, she went into a period of extended mourning, and mourning jewelry was en vogue), and continues to be used today, mostly in cabochon cuts. Jet rates a 2.5 on the Moh's Scale of Hardness, and it leaves a brownish streak when scratched against another surface; jet is also warm when touched, and when heated will smell like coal. The best jet in the world has been mined since prehistoric times in Whitby, England.
See Also: cabochon, Victorian, Moh's Scale of Hardness