Cory Doctorow posted at BB about a 75 yr old jewel thief. My father has a jewelry store, and one of the first rules of helping customers was "one piece at a time". You never, ever took more than one thing out of the case. You kept your eye on it. If people wanted to "compare", then you apologized, but the line was "insurance requires us to have only one piece out of the case at a time". Not sure if that was true, but it made good sense. This woman was clearly exactly what he was worried about. Fast paced switching that ends with her walking out with something. Prevention? Simple. Just only take one thing out. A reasonable person doesn't object to a reasonable security precaution like that. My father even said that those who objected were as likely to be out to game you as not.
Incidentally, those looking for a quality diamond at excellent prices can give him a call. He has a (currently poorly maintained due to a busy B&M store) website at jewelryzone.com, but you can call the 800 number there and get a great deal.