The Gem Mine can be unlocked in the Builder's Base. It requires Builder's Hall level 3 to be repaired.
The Gem Mine, as its name suggests, produces gems, albeit very slowly. These mined Gems are then stored until the player collects them or the storage fills.
Gem Mines undergo a significant visual change at level 3.
When it is ruined, it is a collapsed overgrown cave. There is a gem symbol on a wooden hexagon on the side of the rocks. There are large broken wooden planks covering the entrance, a few scattered gems and a broken box.
When first built, the Gem Mine is a cave-like structure with a wooden arch supported by ropes at the entrance. A Gem symbol is present on the side of the Mine, and a rail conveyor extends out of the cave, into a box containing Gems. Outside the cave is a torch.
At level 2, the Gem Mine's arch and base is reinforced by metal.The grass on the side of the gem mine has disappeared.
At level 3, the top of the cave is replaced by an open roof and a pulley system. More wooden planks are added to the sides of the opened roof, in order to support the pulley. Additional reinforcing metal plates appear at the top of the entrance.
At level 4, the reinforcing metal plates change to a bronze-colored scheme. A second pulley is also added.
At level 5, the reinforcing wooden structure now extends to the back of the cave, and additional bronze plates are added to the back side of the roof.
At level 6, the pulley system becomes bronze.
This building serves no purpose on defense, so placing it on the outside to protect your defenses can be good.
Placing it in the corner can stall your opponent from getting three stars on your base.
This building has low hitpoints.
The Gem Mine is the only building that produces gems.
There are no storages for the gems except for the gem mine due to your unlimited gem inventory.
The Gem Mine was added on 5/22/2017.
Tapping this icon displays information about the Gem Mine, such as Level, Capacity, Production and Hitpoints.
Tapping this icon begins upgrading the Gem Mine to the next level, if you have enough resources and a free Master Builder or O.T.T.O. When the Gem Mine is at maximum level, this icon is not shown.
Tapping this icon instantly upgrades the Gem Mine to the next level, at the cost of one Hammer of Building if you have one. When the Gem Mine is at maximum level, this icon is not shown.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of the displayed number of Gems.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of one Book of Building. This icon is only shown if you have at least one Book of Building.
* The Catch-Up Point is the time at which the newly upgraded Gem Mine has equaled the total production of the mine had it not been upgraded - this does not consider recovering the cost of the Builder Elixir required to perform the upgrade.
Caveat: The Catch-Up Point calculation assumes that the player collects all resources from the Gem Mine without allowing it to reach its storage capacity, as resource production ceases at that point. This is more of a concern at lower levels where the capacity is limited and Time to Fill is relatively short. If the player is unable to regularly collect and achieve the full potential of the collector, the Catch-Up Point is actually shorter and performing the upgrade becomes more advantageous.