Coal :(

Nothing useful, and that is the joke. Coal :( is the lump of coal Santa leaves for the naughty list, a seasonal gag tied to the Christmas event, and the game text sums it up with "Crappy Holidays!".

Do not mistake it for charcoal, the burnt wood your gun powder is made from. This lump goes into no recipe, feeds no furnace and powers nothing. A new player who finds one in December and keeps it for raiding is in for a disappointment.

If one lands in your inventory, the recycler is the only sensible place for it: it comes apart into a little crude oil and some stones. Its row even lists a hand recipe from those same two, but the game files give nobody that blueprint, so you cannot craft one, not even to prank a teammate.