Jet
jet | ˈjet
noun
A dense, black variety of lignite (fossilized coal) that can be highly polished and is often made into mourning jewelry, toys, or buttons, or used in inlays
Jet is related to coal, as part of the Ignite family of minerals, and is considered a gemstone. However, it is not a mineral, but a mineraloid, and is actually derived from wood, transformed over millions of years due to the high amount of pressure it is put under. Because of its natural hardness, it can be highly polished and has been used historically in a number of applications, including decorative inlays, buttons, toys, jewelry, and more. It is colloquially known as black amber and is incredibly dark. The phrase “jet black” refers to the darkness of this particular type of stone.