Crystals are a minor currency. They are obtained through Mining, after clearing a field (removing all 16 tiles). Crystals are used for purchasing Soul Orbs and upgrading their Virtue. The number of Crystals gained through mining (not including Dynamite) can be increased in the Expeditions Lab.
Other sources of Crystals:
- Doing Ship Expeditions
- Participating in Events
- Entering some Codes