The answer to this question will vary from state-to-state. In most states if the marriage does not occur the engagement ring must be returned (including: Iowa, Kansas, Michigan, New Jersey, New Mexico, New York, North Carolina, Minnesota, Pennsylvania, Tennessee, Wisconsin).
In other states, such as California and Texas, who gets the ring will depend mostly on who broke off the engagement. If, for example, the wife-to-be calls off the engagement, she would be required to return the ring. However, in these states the courts may look at, not just at who broke off the engagement, but also, why. If, for example, the wife-to-be breaks off the engagement she might still keep the ring if the court finds that she had a justifiable reason for breaking it off.
The opposite would occur in Montana where the ring is never returned. Some states, including Oklahoma, have no clear law on the subject.
It is important to note, however, that if the couple had entered into a prenuptial agreement, that agreement may dictate what happens to the ring if the engagement is called off. Prenuptial agreements are highly recommended for husbands-to-be in states like California and Montana, particularly if the ring is a family heirloom.
If either the husband or wife was still married to another at the time of the engagement, this will likely change the outcome of who gets the ring. If the husband-to-be was still married when he gave the engagement ring to his new wife-to-be, he cannot get the ring back. It does not matter if he intended to get divorced prior to marrying again because, as long as he is still married, he is unable to legally become engaged to another woman. In this instance the ring is considered a simple gift and not an engagement ring. Gifts need not be returned once given. If, on the other hand, the husband was single and the wife-to-be was still married to another, the outcome may hinge on weather or not the husband-to-be knew that she was married or not. If he new his wife-to-be was still married when he gave the ring, it would be a gift and she could keep it. If, however, he did not know she was still married, he would generally be entitled to have the ring returned.
Another important twist can occur if the ring is given on a birthday or holiday, such as Valentine’s Day. In that case, the ring might also be considered a gift rather than an engagement ring, and again, if it is a gift it need not be returned.
Once married, if the couple gets divorced, the wife keeps the ring and it is not generally considered marital property unless the ring is replaced or upgraded during the course of the marriage. For example, if, after the wedding, the couple upgrades a one-carat diamond engagement ring to a three-carat ring, the ring may then be considered marital property and factored into the overall division of property upon divorce.