I have never lost my diamond. I have, however, bent prongs a few times, so I (like SusieQOH) had my diamond reset in a bezel setting. Much more secure.
My DH and I love to metal detect. We hit the beaches in the nice weather. DH hunts the water, I hunt the sand. We have found and returned two engagement rings (one was lost on their honeymoon!). We have also found 5 or 6 engagement rings that we haven't been able to return. (Yes, we notified the police that we found them, listed them on the lost items websites, etc. but no one ever claimed them).