Jet

Jet is an organic gem, not a mineral: a black, coal-like rock (a form of lignite) formed from fossilized wood. Individual pieces of wood sank into water, were buried in organic-rich sediment and were coalified, giving a tougher, more uniform material than seam coal that can be carved and polished to a bright luster. It is light in weight, leaves a brown streak and has been heavily imitated by glass and plastics.