Lump of Coal.
In-game description

Coal is a material in The Long Dark.

Coal is the only fuel source that can be used to heat a furnace up to 150 C for forging. Unlike wood, coal can only be added after 30 minutes since the fire started.

Coal has a fire duration of 1 hour and adds 20 degrees celsius to the fire. This makes it the most weight efficient fuel in the game, both temperature-wise and duration-wise.

Coal is mainly found in mines, randomly laying on the ground with a respawn cycle of a few days.

It can also be found inside the Riken, inside the crates near the furnace, in the Old Spence Family Homestead, and in caves with interiors, as well as a few exterior caves.

Fire Starting
Starters Cardboard matchesFirestrikerMagnifying lensWood matches
Tinder Birch barkCat tail headTinder plugCashNewsprintNewsprint rollStack of papers
Fuel BookCedar firewoodCoalFir firewoodFirelogReclaimed woodStick
Accelerant AccelerantLantern fuel