I though natural too, the general shape doesn't appear to be worked at all. Not sure about the notches, is there any evidence of working inside them? Maybe used as an anchor?
Good guess, but that seems like a lot of work for an anchor, I don't think so. I believe it's natural, but I am stumped. I know who could ID it, but I would have to send him photos or post photos to the forum he frequents. Here is a 30 pound sandstone anchor from RI.