If it is a forced breakup, precipitated by his misdeeds, I'm keeping the sucker. If I end it, I may give it back unless it comes from Tiffany or some kind of class jeweler.
Nobody's taking a Tiffany ring from moi.
On a side note, I was on another forum where a woman was asking advice. Her fiancee gave her a CZ ring and apparently lied about it. She discovered it was fake after she nicked it and her jeweler told her.
If that is the case, the jerk gets the ring back and it's off.