I bought a mystic fire ring in St. Thomas at one of the shops right by the pier. I really wanted one, and the girl who was showing me the rings was nice but a little pushy. I bit at the sales talk, and bought myself a nice ring. It looked beautiful in the store, but it needed to be sized for me, and it was included in the purchase price. It would've probably cost me 10 dollars a size up ( maybe 30 for mine) I paid for my new ring and was given my receipt to pick it up after my excursion. I came back and picked it up, it fitted nicely, I took it back to my stateroom and it didn't seem to be as sparkly as the one I saw in the store before purchasing. I think they may have switched stones on my ring when they sized it for me.
So if you buy a ring take it right then and have it sized at home, so you can be sure that it did not get switched.