A mineral charcoal
In the same way we make charcoal out of wood, in the ground we found coal, an oily dark kind of stone that, if baked it in a similar fashion turns into coke which can be used as a combustible in the same way we use charcoal. Mastering this process would allow us to get combustible without the need to cut down entire forests!
-Have metallurgists or miners in a region that has coal
Coke gives access to the ultimate fuel resource in Ymir, Coke.