Gems

Gems are found in chests and granted when certain milestones are met. During the game players will acumulate gems, which do not reset on death. Instead, gems are kept and used in the Upgrade Shop selectable from the main menu.

Also, gems are outrageous.

Chest Spawn
When a chest contains gems, the player receives 5 gems, plus a number of gems equal to the chest value * 2 (see chests).