Heatstone is a mineral mined by the dwarves of the Dwarf Kingdom.
The dwarves use this curious metal like coal, and their refineries and smithies consume vast quantities of them. Thus, one could say that heatstones are a central part of dwarven life. Heatstones become the replacement of wood and serve the subterranean race in all manners. It is mined primarily for heating, illumination and cooking.
Heatstones are a special kind of metal. If one struck them with a very hard metal mithril, at the barest minimum they would produce intense heat.