Speak to a lawyer.
And to all of you just flat out saying "no, it's a gift". Lord I hope you never have to deal with a broken engagement because it isn't that clear cut. There are a lot of states out there that have "conditional gift" things when it comes to engagement rings. That means that the ring is given as a gift under the condition that the couple actually gets married. AND! In some cases, if the ring is not given back, you can go after the cost of the ring.
However, the asker of this question still won't know what HIS chances are until he speaks to a lawyer.
Oh, but I gotta say, you can't squeeze water from a stone. Even if you win, there's still a chance you never see that money, because if she doesn't make enough they can't garnish anything, blah blah blah....anyway, just go talk to a lawyer, find out if it's worth your time