The crystal mine is a mine in the Heroes of Might and Magic games that gives the kingdom crystal each day. The hero gains one unit of crystal each day they control the mine.
In Heroes III and Heroes V onwards, the structure is referred to as a crystal cavern. There is also a crystal cavern structure in Might & Magic: Heroes VI that produces blood crystals. In Might & Magic: Heroes VII, the crystal caverns produce dragonblood crystals.
Might & Magic: Duel of Champions included a card version of the crystal cavern.
Crystal cavern (Duel of Champions)