Charcoal was a highly porous form of amorphous carbon created by the destructive distillation of wood.
When Spock observed Kirk's collection of raw materials as he evaded the Gorn captain in 2267, he noted Kirk's attempt to create a makeshift cannon. Putting the evidence together, he surmised that if Kirk were to locate a charcoal deposit or ordinary coal, he could complete and successfully utilize the weapon he was apparently fashioning. (TOS: "Arena")