Oooh! Sparkly gems! And all of them are green too!
Gems are the main currency in Battleblock Theater. They are large green collectibles that float in the air.
Usually levels have seven gems in them but this depends on the level. The player only has to collect three of the gems 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. After buying more prisoners the cost will increase to 15, 20, 25 and stop at 30.
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.