Found in the river, it's smooth all over except in one spot. I had originally posted in a different forum and though I had posted it here as well, but apparently it didn't post here.
That looks like battering evidence to me. It was used to crush or pulverize something--like a pestle. I'd say it's a naturally shaped piece that was found and used as a pestle.
May be naturally formed, but I agree that it could've been used as stated above. The tapered end could've even been fitted into a socketed handle like a celt and used to beat hell out of something.