Jet is a geological material not consiered a true mineral, but rather a mineraloid derived from decaying wood under extreme pressure, and thus organic. The English word-name "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 color and metallic lustre. The adjective jet-black is better-known perhaps than the substance from which the descriptive phrase derives.