Jet

Jet was a variant of bituminous coal that could be cut into fancy stones.

Description
Jet was a deep black gemstone that was hard enough to take facets and was often used in simple pendants or as part of a larger setting. In the wealthy commercial centers of Amn, Calimshan, and Sembia, jet was worn to represent sorrow and mourning. This custom was also practiced in Waterdeep and Westgate. A typical specimen had a base value of 100 gp.

Powers
Jet was the material of choice for magic jars and could be made to release the captive life force within by means of certain treatments, or accidentally trigger the release by casting specific spells in close proximity to the magic jar. If this were to happen, a person or creature near the magic jar would have their life essence sucked into the jar and the captive entity would be released to inhabit their body. Being in possession of a magic jar rarely gave any control or influence over the entity stored within.

Legends and Rumors
The legends of jet being used for magic jars were true, and stories about irascible wizards or dire magical beasts popping out to possess the body of the hapless victim gave jet a bad reputation. Jet gems and jewelry found in treasure troves were generally viewed with great suspicion.