Gems

Added in v2.3, Gems add a whole new dimension to the game. You can obtain Small Gems from tourists with a above their heads. Tapping on this will open a window asking you to give the tourist a number of, and  in exchange for their gem.

Gems can then be applied to buildings to boost their performance. There are two ways to do so.

The first method is:
 * 1) Select the gem you wish to apply from the Warehouse menu, Inventory tab
 * 2) Tap the "Place" button
 * 3) Select the building you want to apply it to.
 * 4) If all is to your liking, tap the green tick to confirm the application of the gem.

The second method is:
 * 1) Tap the building you wish to apply a gem to
 * 2) When the building info window opens tap on the empty gem slot.
 * 3) This will let you select one of your available Gems to place there.
 * 4) Finally, tap the "Place" button to apply the gem.

Initially all buildings have only one slot to apply gems to, but for €15 you can open a second slot. This means you can also boost the building in another area. Note that buildings can have only one gem from the same category, even when it has two slots. If you want to change the applied gem, open the building's info window, tap the gem and pay $5,000 to release it.

Only Small Gems (the rhombus-shaped ones) can be obtained from tourists. To get the better Gems you will have to set Albrecht to work. Below is an overview of the Gems, their costs and their effect. Small gems can also be purchased directly from the Warehouse at 20 € a piece.

Gems cannot be placed or removed while a building is under construction or repair. These actions can only be accomplished on healthy structures.

Bug note: There seems to be no check whether you have enough resources when buying Gems from tourists. Therefore it's possible to have a negative number of resources after the purchase.