Dracanthracite

"Dragons... they shit gold. S'why they lairs is filled with it. Think it, they ain't carry off gold. They carry off whole cows n' maids. Time pass, an' there's gold. Psience" - Yiddle the Spellthief
Dragon (aka Tarak) shit isn’t actually gold… but it’s KINDA like gold. The substance allowing dragons to breathe fire also comes out in their shit. So dragon shit can be burned as fuel that is better than any fossil fuel. Thereby, over time, an economy based on raiding dragon’s lairs to get Dracanthracite, effectively magical “super coal” has sprung up. The dragons have gill like structures that pull prana, or ambient magic, from the air. An organ in their chest turns this empyrea into biofuel, similar to photosynthesis. Some is used for living, some for fire breath, but the rest is simply excreted. As the stool hardens, it leaves a soft, consumable stone that can be burned to release the stored magical energy.
Due to a quirk of the empyreal digestion, a dragon fed on blood that is of only one soul which has no marring of its identity excretes the most pure Dracanthracite. So, sadly innocent, virgin blood makes for the best fuel. Landlords tell us the dragons even DEMAND this type of sacrifice.