Unless you're a gemologist or mechanic, I think dealing with jewelry is a lot like used cars - how would you know a good deal or not? If you can't semi-trust the person you're working with, you'll probably leave the transaction thinking, "Did I just get hosed?"
Which brings me to Mr Bernstein: I actually found him through his Yelp reviews, he's an estate jewelry buyer, and as far as I can tell, he's more interested in jewelry than taking advantage of people. Although he certainly could. And who you gonna call, Gembusters? He studied my ring for a bit & declared: "You probably could get about three thousand for this at a vintage show, and dealers will pass it among themselves for fifteen hundred. I'll give you eight." He didn't charge me for the appraisal. I calculated how much more time & effort I wanted to put into this, and decided to take his check. You may have played it differently.
I think he has an Arboretum office, but I met him at the Benold's on Anderson Lane, where he was appraising customer's jewelry for the day. Make an appointment with him through his Yelp or web page, and if you can, text a photo - it may pique his interest: "That looks interesting, can you come by Benold's?" He responds quickly, and he genuinely is a cool guy. If Mr Bernstein had offered me a fairly recent Porsche at a decent price, I might have considered it.