Coal

is a readily available.

Uses
Coal can be placed in any Fuel Dock on a handheld Lantern, Smithy, Spotlight or the various ceiling lanterns scattered in Overworld buildings. Coal burns hotter than wood so cooks food and smelts ore faster, but doesn't burn as long so isn't advisable for lanterns or spotlights.

Obtaining
Coal must be Mined from black Ore Veins. These can be found primarily in the Mines, but some coal and other Ore Veins spawn in the Mountain Pass. Coal can occasionally drop from slain Turabada.

The ore vein for coal looks like: