Wouldn't want to get hit in the head (or anywhere else with that)! Is the side opposite the flat side sharpened? Can't tell in the pic. Beauty, though. How big?
I have seen some creeks that had a LOT of stones like that and they were made by seasonal water flow carrying pebbles/grit. Is the shaft of it triangular If so the leading thin edge would point upstream. Wish I had pics, gave them away. The ones I found last were from the Genessee River in NY