15 weight units
Coal is obtained from the Mine or through trade. It is an alternative fuel resource to firewood. This can reduce the need for so much forest area, at the expense of having more miners. Coal can be used by the blacksmith to make steel tools which last longer than iron tools.
The downside is that trade is a very sporadic source: coal can only reliably be obtained from a mine. While the player can remove the wooden or stone part of the structure, they leave a heap of dirt or a pit in the ground, and the land is rendered permanently unusable.