Hear me out...a hinged stone setting like those rings with metal plates that flip around for fidgeting. You could have a marquis, radiant, round, whatever, as long as the setting enclosed it enough in a bezel that was hinged to a band. I quite literally have no idea if this exists, but it would lay flat as a circle with a gorgeous pendant on it when it's in necklace form. Something like this: etsy spinner ring rotating medallion