A black gemstone which is actually fossilized wood -- not quite the same thing as coal, but it will give off a smell similar to burning coal in extreme heat. The most significant source is England but it is also found in many other areas. Jet was very popular during the Victorian era as mourning jewelry (and less popular in Western cultures in the 20th century because it continues to be associated with death and sorrow by the superstitious). Native American and Asian cultures don't have this association and it is more commonly used there. Though jet polishes up quite well to a high shine, if not cared for it can dry up and split.