Celestial foam is one of mid-game Stones. If the player has more than 64 Hell Gems, each Celestial foam destroys 64 of them and annihilates the moment it's harvested. These reactions continue until the amount of Hell Gems player drops below 64.
Hell Gem threshold can be extended up to 512 by building a Hollow rock research site.
Notes
- Since Celestial foam reacts with Hell Gems immediately upon being harvested, it's not possible to quickly store it by building a machine. However, it's still possible to do once all Hell Gems have been consumed, then the player can stop harvesting Celestial foam and set up a separate mine for Hell Gems, which can be stored the same way later. Then the cycle can be repeated.