Coal is a resource found deep in the ground on Terran, Barren, and Exotic. (Found on surface of Arid) It is black and resembles the shape of compound. It is used as a more powerful power source in a generator. It doesn't require the drill to mine, it can just be mined using the terrain tool.
WARNING: When mining for coal, make sure you bring lots of tethers (4-6 stacks). DO NOT DIG STRAIGHT DOWN TO FIND COAL! Mine at an angle to avoid falling to your death!
Good luck pioneers! And may the Cosmos favor your journey!