In general, I see nothing wrong with wearing jewelry from exes. Personally, I burn everything from exes. But, if it's pretty valuable and someone cannot sell it, I can't find fault with someone else keeping said jewelry and wearing it.
The only item I would question would be an engagement ring. If he wants it back and has been a complete ass, you can either sell it or wear it for a while just to piss him off and flaunt the ring in his face. If you sell it, take yourself on a nice little trip, etc. If it just brings too much emotion, then still sell it and spend the money on something REALLY nice for yourself.
