Tales of Tolgard
A deep black gemstone, this fancy stone is a tough variant of bituminous coal that can be facet cut and displayed either as a pendant or inset into a larger setting. It is mineralized wood that has decayed under extreme pressure. It comes in two varieties, hard and soft. The former is associated with salt water and the later with fresh water, although both have compression.
It is the stone of mourning and sorrow in wealthy cities (such as those in Talisman, Deepwater, and Glacerin), and remains a preferred material for magic jars, a use contributing to its fell reputation. Certain treatments of a jet stone (or specific spells cast too close to one) may well unintentionally free a furious, long-imprisoned wizard or strange magic-wielding beast from its depths or summon a wizshade to the spot. Some such imprisoned beings can use their magic in limited ways to try to bring about their release but possession of their prison gemstones rarely gives one any influence over them.