Coal is one of the 31 Rare Resources in the game. It becomes visible to a nation in the Industrial Age (until then it appears as a rocky patch). Coal is distinguishable as a fissure with lumps of dark coal in and around it.
The gather rates from all rare resources can be increased in various ways, as described in the main article.
The cost reduction affects all unit, building and technology cost, making this resources' bonus a quite powerful one. Amongst other things, it helps reducing cost for Commerce Research and the future tech Artificial Intelligence.