Coal

Coal is a common item obtained by mining a coal rock at level 30 mining, granting 50 mining experience. Two or more coal ores are required for making most bars in the smithing skill.

For a list of coal rock locations, see coal rock locations.

Smelting
In order to smelt certain types of ore into bars, multiple coal ores need to be smelted with the single non-coal ore. Using the ore that requires coal on a furnace will result in a bar being produced, assuming that the player has a sufficient amount of coal in their inventory.

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Trivia

 * Coal is not an ore; ore requires extraction to obtain the element, whereas coal is found in its usable form.
 * As a joke around Christmas time, some players give coal to other players saying it's because they have been very naughty this year, e.g. killing innocent guards, picking people's pockets, etc.
 * It's possible for two players to receive coal if they are mining it at the same time.
 * The examine text states "hmm a non-renewable energy source", a reference to coal in real life as it cannot be replenished once extracted.