This doesn't answer your question, but keep in mind that any insurance reimbursement reduces your casualty loss. If the ring was worth $15,000 at the time of the theft (however you determine that), and the insurance pays $12,000, your casualty loss is $3,000. After you subtract $100 and 10% of AGI, you might have no deduction at all. If you think the value is in the range where there's likely to be no deduction, there's no point in losing any sleep over how to determine the value.
