A mine represents a worked holding. It could be a small but profitable shaft mine, a series of underground tunnels but it could be something as small as a panning operation.
Upon taking a mine, you must choose one of Green Iron, Tempest Jade, Weltsilver or Orichalcum. A mine produces 12 Ingots of the chosen material.
It is possible to obtain additional materials from a mine during any given season. You may spend 6 crowns to have the mine produce an additional 5 ingots of the same material as it would naturally produce.
A mine can be upgraded using mithril obtained from the Bourse. Each time a mine is upgraded, the owner receives an additional 2 ingots from then on.
To upgrade a mine requires Imperial wains of mithril equal to the level the mine is being upgraded to. So the first upgrade of a starting level 1 mine costs 2 Imperial wains of mithril, from 2 to 3 costs 3 Imperial wains, etc.
A mine can be diversified to sacrifice production of one material for another. To diversify a mine requires one Imperial wain of white granite and one Imperial wain of weirwood. Each time a mine diversifies, the owner chooses a different resource from the list for mines; the mine produces 2 ingots of the new material at the expense of 2 less of the original material.