Jet

Jet is a type of lignite, the lowest rank of coal, and is a gemstone. Unlike many gemstones, jet is not a mineral, but is rather a mineraloid. It is derived from wood that has changed under extreme pressure.

The English noun jet derives from the French word for the same material, jaiet (modern French jais), ultimately referring to the ancient town of Gagae. Jet is either black or dark brown, but may contain pyrite inclusions which are of brassy colour and metallic lustre. The adjective "jet-black", meaning as dark a black as possible, derives from this material.