Well I will say this Rock. You can not say a single broken rock, no clear undoubted evidence of knapping that you see right off, is an artifact worked by man without some sort of surrounding context that supports that. I can find you rocks all day long that appear to have been worked, fit perfectly in the hand, have thumb and finger holds, but they're just rocks, because there's nothing else there to support prehistoric human activity. They did make quick throwaway tools, but there will be several, not just one. That's how we discovered a previously undocumented site in SW Oklahoma. We found a chip, a flake, a few more, note that they look like cutting tools, keep looking, and sure enough it appears to be a prehistoric kill site where they stopped to butcher and process, stayed a few days, then moved on. Best advice I could give to people who think they've found an 'Indian artifact', keep lookin around. See what else is in that spot.