Coal

is the best Fuel Item in the game.

Uses
is the longest lasting Fuel Source available in the game. It can be placed in any Fuel Docks.

Obtaining
must be Mined from Ore Veins with black stripes. These can be found in the Mines as well as the Quarry just outside of the Mines, as well as scattered about in caves within the Overworld.

Related

 * Fuel