Finite Stones

 a limited-time currency released on June 14, 2016 as part of the Superheroes 2016 Event. It used to upgrade the player's heroes. When an upgrade takes place, the target increases its maximum life hearts by 1, additionally it fully recovers any prior heart damage making it instantly available for fights.

It can be obtained,
 * As one of the rewards from Battles.
 * As one of the rewards from the "First Win of the Day", which is the first Felon Battle played that day.
 * Purchased 5 per for 1 Donuts, but only if the player to upgrade their heroes without enough  remaining.

US$