Who can I see about an ex fiancé returning an engagement ring to me?

In New Jersey, an engagement ring is considered a gift in contemplation of marriage. If there is no marriage then the ring should be returned to the giver and it does not matter who breaks off the marriage. If the marriage does occur, then the ring would not be returnable. So to answer your question, if the ex does not return the engagement ring you should hire an attorney and go to court if it is financially justified to do so.