a good rule of thumb with stones of particular shapes is, if you are finding several that have a similar shape, but question if they are artifacts, then they aren't. I know i am not making much sense but look at it this way...say you find a rock (like that first one) that really looks good but you question it...and then a few more pop up with the same basic shape but are in no way an artifact...that lets you know the first one isn't either.