Jet is a geological material considered to be a minor gemstone. It is originally a wood product called lignite, squeezed and heated by geological processes. The older definition of mineral did not apply to organic material. Therefore jet was not called a mineral, but instead a mineraloid. However, this view is changing, as so many crystalline materials have organic origin.
The English noun "jet" comes 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, comes from this material.
- Discussed in part II of: L.B. Railsback Definitions  and 
- Oxford English Dictionary 1989. 2nd ed, Oxford University Press.
- Pye K. 1985. Electron microscope analysis of zoned dolomite rhombs in the Jet Rock Formation (Lower Toarcian) of the Whitby area, U.K. Geological Magazine, 122, 279-286. doi:10.1017/S0016756800031496