is it o.k. for the police to give back jewelry evidence, to the owners, before the defense has a chance to get an appraisal?

The short answer is yes. Unfortunately there are cases that talk about the value being what the person says it is. That said, your lawyer make a request to have the jewelry apprasied even though it has been returned. It can request that the owner take it for an appraisal & that you pay for it. It can even go as far as suggesting an appraiser and a time for the lawyer to meet with the owner at the appraiser's office (or have the owner return the jewelry temporarily to the prosecutor so that the prosecutor can meet with your attorney at an appraiser's office.)

It is not unusual for the police to return stolen property so that the owner is not without the property during the time the case is pending. (That would be punishing the owner.)

The previous advice/answer you have been given is sound. The authorities are permitted, with district attorney/prosecutor approval to photograph the evidence and even utilize the photographs at trial. Your attorney via discovery will obtain photographs of the ring as well as appraisal information. He/she can try to use that to get your own appraisal. If those materials are insufficient than there are procedures whereby the state attorney, and your attorney can meet with the ring at your appraisers to have an appraisal obtained.