Coal

is the longest lasting Fuel in the game.

Uses
Coal can be placed in any Fuel Dock on a handheld Lantern, Smithy or the various ceiling lanterns scattered in Overworld buildings.

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.

The ore vein for coal looks like: