Jet is not really a mineral. It falls under the classification of mineraloid since it is organic in nature. When dying wood is under a tremendous weight from water, that pressure transforms the wood into jet. If it is soft jet, the water was fresh. If it is hard jet, the water was salt water. The jet we have today is from trees that died millions of years ago. It is either the color of the wood or the decay, so brown or black. Jet black is the name for the color that is the same as black Jet.