The Mine becomes available when you reach level 27. To restore the Mine, tap on it and select Repair.
When you restore the Mine, you unlock the ability to extract ore and amethysts with special items: pickaxes, dynamite and powder kegs.
Items for the Mine can be obtained from chests on the island and from reward chests for sea expeditions on the Ship. These items may also appear at random in rewards for reaching buoys in regattas.