A negative Gems/Diamonds balance may occur if a refund is requested on a used Gem/Diamond Pack offer. When the refund request gets approved, the respective in-game currency is automatically deducted from the account.
If no refund was requested, the problem might be that the payment didn't go through, and an automatic refund was issued, resulting in the automatic revoke of the respective Gem/Diamond value. If that’s the case, please contact Google or Apple to check your payment status.
For Google (Android) devices:
For Apple (iOS) devices:
If you need any further assistance, contact us with a screenshot of your order receipt.