No matter who calls off an engagement, a huge issue afterwards is always the status of the engagement ring. The giver wants it back, the receiver often wants to keep it or sell it; who technically owns it?
The courts in Pennsylvania have ruled that until a marriage takes place, the ring is a conditional gift to the recipient. This means that if you give someone an engagement ring because you intend to marry them, the gift isn't complete until you actually get married. If for any reason the wedding doesn't take place, the condition on which the gift was based (a marriage) doesn't occur, therefore the gift doesn't occur. The giver is entitled to have the ring returned.
Once a marriage takes place, the ring becomes marital property, subject to equitable distribution. Neither party owns it outright at that point, it has become an asset of the marriage.