Jet is a form of Carbon. It is not a crystal at all, but a precursor stage form Peat to Coal which has just barely been pressurized and heated enough to fully change it from organic to mineral. If left in the ground, it might actually metamorph into Diamond. It is just hard enough to cut and carve and can be polished to a fine, soft luster for use in jewellery.
Jet is flat Black, and for this reason it was popular in Mourning Jewellery in the past, particularly in the Victorian era. While many other Black or dark stones have also been used in mourning jewellery, including Onyx, Black Coral, Obsidian and Smoky Quartz, they had a shine when finished. Jet was a favourite of Queen Victoria, because it does not reflect light…. she saw it as the ultimate symbol of her sadness.
Jet is found in England, Spain, France, and the United States. Because it is relatively soft; care must be taken with Jet jewellery. However, its softness also means it can easily be worked into carvings, cameos, various sorts of beads, and other interesting shapes. The easier a gem is found and the easier it is to work greatly affects a gems price. Therefore, Jet is a rather inexpensive gem, and yet a timeless classic–as you know, black goes with everything.