In the official podcast with Adam and Eddy they mentioned the ring: they said that while Rumple is swag-tastic, seeing if the ring has greater significance is something we could watch for. (paraphrasing) so SpinningGold might be on to something!
I thought it was an Opal..which legend has it, can make the wearer invisible. Sounds like something Robin Hood would try to steal from Rumple in 2×19. Just my guess!
Yes looking up close (which wasn’t possible until now) I have started to wonder too. It could be, but I miss the flecks you usually see in opal, like someone threw oil shine glitter in it… It still could be either (or more like it, a piece of paste to resemble either one 😆 )