What law applies when one party lives in a state where the ring is a conditional gift and the other lives in a state where it is based on who calls it quits?

What are the two States? You would have to look at case law.

Kentucky and Indiana...
Thanks

I'm sure you know there are different decisions, varying opinions, pretty much all over the place. It appears that this would be a matter settled by a Judge following trial if it comes to that.

In some States the promise to marry binds the contract; in other States the marriage has to actually take place.

I would say the person who sues in whichever State will have a definite advantage in that State.

The majority of the cases I find seem to lean toward the "promise of marriage, no marriage, return the ring" idea, fault aside.