The iron mine is locked and cannot be activated until the Monarch activates the stone mine. The activation does not require gems, but only coins. Once paid twenty coins, the mine is "self-repaired" without the help of builders, and gives access to:
Without this tech, the Kingdom structures can only reach the stone tier. After paying for this tech in the fourth island, the Monarch is allowed to upgrade them to the iron tier on any island, even if the Monarch loses the Crown.
- As seen on this by AhriFangirl.