I think they are all pretty much 9/16ths, maybe 11/16th, but I will measure them tomorrow,,,,Galen, will you look at my other post with the blue/yellow/green marble in it, thought you might have an idea...I am 99% it is not a newer marble it came in the lot with these ones and they were all mostly Akro, Alleys, and Peltiers; a few Masters, & a couple of Christensens, but that is all. (ok, I realize that is more than a few...lol)