Can't say. I had one of just about the same size. Some said what I had was man made and one fella, a mine engineer said it was a wonder of nature and a process of natural erosion. I gave mine to a customer who collected vintage marbles, as he thought it was very unique. Those things seem to always fall in to the "difference of opinion" category. I'll be here on the fence waiting to hear what others say.
Welcome to TNet. CBG, a cannon ball expert on this forum will no doubt answer you shortly. I don't know what it is, but don't think it's a cannon ball.