Jet is a type of lignite, a precursor to coal, and is considered to be a minor gemstone. Jet is not considered a true mineral, but rather a mineraloid as it has an organic origin, being derived from decaying wood under extreme pressure.

The English noun "jet" derives from the French word for the same material: jaiet. 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.