If a jewelry store lost my engagement ring, what are my rights?

It is not that easy to effectively translate sentiment into a lawsuit specific amount. You need to understand that the underlying transaction seems to be one in contract (i..e, you had the ring there to be sized or cleaned or updated). It wouldn't appear that the likely culprit is loss but possible theft internally by staff or simply lost in transit if it was shipped somewhere for additional work. You should (if this was a particularly expensive ring) consider talking to a lawyer and see if the lawyer can discuss the matter with the store and the store's insurance company about a replacement ring (same or better than the one you had) and the emotional distress caused on what would be a contract breach. Typically emotional distress is disallowed for such contract breaches but you may be able to get lucky with this scenario.