Jet is produced by wood that has been exposed to extreme pressure. It is a form of coal. 
It has been in use since the neolithic period for jewellery and small ornaments. It became particularly popular for use in mourning jewellery during Queen Victoria's reign.
<a href="//commons.wikimedia.org/wiki/User:Sudzie" title="User:Sudzie">Sudzie</a>
<span class="int-own-work" lang="en">Own work</span>