Civil Code Section 1590 allows you to recover whatever seems just. You will have to sue her in small claims and see what happens. How did she get the wedding band if you are not married yet? If you did not give it to her she has no legal right to keep it.
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Gifts are gifts and aren't subject to revocation, and I think it would be hard for you to prove that these rings weren't gifts to her, which she should be able to keep. There's no cause of action for failing to marry someone, and you can't argue that she was only allowed to keep the rings if you had married her. You also can't claim that you lent her the rings. I don't know of any theory that would be successful.
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It is unclear from your question who broke off the engagement. If you broke off the engagement, then anything you gave your ex as a gift in contemplation of marriage is your ex’s property. If she broke off the engagement, or the engagement is broken off by mutual consent, then you may recover any gifts in contemplation of marriage that the court deems just.