A new type of mine, ‘emerald mines’.
You can extract coins and experience at the same time, it is infinite, but only a few villagers can work.
An alliance with the Muisca will make these mines more productive: Renewals to the original content and its first two expansions - #73 by OperaticShip743
The emerald mines could be on the same maps that the Muiscas could potentially be on, although as the graph indicates, they could also be on other continents.