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.
Hides Black bearDeerMooseRabbitWolf
Other Crow featherGut
Medicinal Old man's beard lichenRose hipReishi mushroom
Sapling BirchMaple
Renewable Cedar firewoodCoalFir firewoodStick
Nonrenewable BookFirelogReclaimed wood
Renewable Birch barkCat tail headTinder plug
Nonrenewable CashNewsprintNewsprint rollStack of papers
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