In my current D&D campaign, I'm an artificer. Part of our role playing is me inventing numerous weak magical trinkets for my allies. I like to write the item descriptions for these in a WotC-style language to make them feel more real.
Currently I'm looking for a vanilla mineral that generates constant heat. This feels like something that would have been described at some point in the sprawling Dungeons and Dragons lore, but my Googling's turning up nothing.
What mineral from any D&D published lore is described as generating heat?
As an example, I'm currently substituting "two small pieces of infernal jadeite, pulled from the elemental plane of fire," which I made up. There aren't really any limitations on this; the more exotic this mineral the better, and the method by which I need to acquire this mineral doesn't really matter. The only consideration is that ideally it would be quite small.