From Wikipedia
In the United States, engagement rings are considered "conditional gifts" under the legal rules of Property. This is an exception to the general rule that gifts cannot be revoked once properly given. See for example Meyer v. Mitnick, 625 N.W.2d 136 (Michigan, 2001), finding the following reasoning persuasive; "the so-called, "modern trend," holds that because an engagement ring is an inherently conditional gift, once the engagement has been broken the ring should be returned to the donor. Thus, the question of who broke the engagement and why, or who was "at fault," is irrelevant. This is the no-fault line of cases."