Hard to get a sense of scale but looks to be about an inch dia. Its a brass rivet used in ocean going wooden shipbuilding. The pin is driven through a bore between two wooden beams, be they frames, futtocks, keelson or other framing timbers. They are peened at the ends. They are often used in conjunction with treenails made of hardwood, of which there is a treenail visible in the timber close to the rivet. Too bad the timber has been sawed, the toredo worm, or sea termite, eaten hardwood with the green patina rivet plus the treenail would have been a nice artifact to keep intact as it shows two different shipbuilding fastening techniques.