Coal

Coal is the most common of all of the ores and the veins are usually found between lower dirt (not surface) and rocky light dirt. Coal can be used for making many items, especially those needing the forge or the kitchen to be crafted/cooked.

Acquiring
Coal nodes can be found in the first three earth levels (see the diagram at the right): the soft earth, the rocky earth light and the rocky earth.

More specifically, considering the world surface as the 0% and the lava level as the -100%, coal can be found:
 * between -6% and -53% in all worlds and the Underground Biome and
 * between -6% and -63% in all biomes, except the Underground Biome.

Drops
Some creatures drop coal after dying, like: (10% chance) (10% chance) (10% chance) (10% chance)

Crafting
Coal can be crafted after unlocking the Railroad Construction category on the tech tree (not the Substance Transformation, meaning that you can craft it at all worlds, not just at the Underground World).