Bituminous coal is found in veins in sedimentary layers and is one of the two mineral sources of fuel. When processed at a smelter or magma smelter, one unit of bituminous coal produces 9 units of coke. If done at a regular smelter, this processing requires one pre-existing unit of fuel (either charcoal or coke), leaving a net production of 8 fuel.
Bituminous coal is flammable, and obviously is not a fire-safe material - if exposed to fire or magma, an item made of bituminous coal will burn for the better part of a year before wearing away. Exposure to water (including rain) will extinguish it, unless it happens to be stored in a bin.
Bituminous coal is not the same as "refined coal" or "coal", though it is directly related.
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Of all the fallacies of the humans probably the most laughable is "Old Eartherism". Some humans have estimated the world's age from the ludicrous 6000, to the unimaginable 4.5 billion years. As "proof" they point to the bones of giant lizards buried beneath the soil, and coal, which they claim is the byproduct of the lizard's decay. Dwarven history extends back to 1 year after the creation of earth, and coal is amply documented even then. Dwarves are also well aware of how those monstrous lizards' bones ended up underground, as well as *exactly* what they decay into. They don't bother to correct the humans because they think they're cute when they're wrong.