Gems are the main currency in Battleblock Theater. They are large and green and float in the air.
Most levels have seven gems in them, although the player only has to collect three of them to be able to exit the level.
The amount of gems the player collects is one of the three ways that the player is scored upon completing a level, the other two being how fast they completed it, and whether they got the golden ball of yarn or not.
Gems are used to buy prisoners (playable characters) in the gift shop, each prisoner costing 10 gems when the player starts off, and progressively getting more and more expensive as the player collects more prisoners of that particular head shape. After buying about 5 more prisoners of the same head shape, the price of that shape goes to 15. After 5 more prisoners, it goes to 20. 5 more increases the price to 25, and another 5 increases the price to 30. The last of the prisoners cost 30 each.
The gems are kept for safekeeping in the massive vault, and Hatty hordes it all. The Cats do not like this, and eventually begin to revolt, beginning around the time of Chapter 6. Gems can be seen littered about the floor in the background.