Edit:
In v1.0, pink diamonds didn't technically have any functionality. Their primary purpose was to unlock the other queens in the game.
In v1.1, pink diamonds are currently used to unlock the other queen, Zafira.
However, starting with the v1.2 update, pink diamonds will no longer be used to unlock queens. Instead, they will serve as an in-game currency used to purchase items from the Devil's Shop, benefit from item effects, and access other gameplay features.
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