While in the ground, coal appears as black, squiggly ovals, similar to flint. As an item in the inventory, it appears as a round pentagonal shape.
Where Found
Deposits of coal are common underground, where stone is found. It is more common near the surface of the world. Prior to version 1.4, coal could be warped in from the portal.
Crafting
Coal is required to convert an iron ingot into a steel ingot in a furnace or electric furnace, taking two in a normal one and one in an electric one. Prior to version 1.4, crafting steel required five coal.