Jet is a type of lignite, a black organic rock that forms when woody material was buried in sediment and coalified. So, in essence, Jet is fossilized wood, a less brittle type of coal. Since it has an organic origin, it’s not a mineral. However, it is considered a gem, like amber and pearl is. Jet will burn like coal, releasing an oily odor, whereas plastic imitations will have an acrid smell. 
 
Jewelry made of Jet dates back to Neolithic times in Ancient Britain. Later the Ancient Romans popularized Jet for ornamental use and its 'magical properties'. Taking after Queen Victoria, during the Victorian Era, Jet jewelry became a popular to wear while mourning.
 
Clean your Jet with a mild detergent, using soft brush and warm water.