I first thought it might be a bird band, but, I don't see a seam anywhere. Maybe it's just a spacer of some sort. There are some strange symbols (Greek?) around the outside. Nothing on the inside. O.D. is approx. 5/8" and I.D. is 1/2". I don't know how to post these symbols, so, I'll just have to describe them as best as I can. There are two groups of 3 symbols with a dash in between the groups. No telling which group is first as everything is evenly spaced with the dashes. One group starts with an up side down V, followed by an X, then an A. The next group starts with an A, then a backwards 7 and then a triangle (delta?) Found at the beach if that helps any. Let the speculation begin!