I was about ready to ask the same thing. My son found a rock that looks just about like that except that it is smaller. It is like a block and you can see where the top surface meets just like yours. Our rock was also found in a creek in Missouri.
i found obsidian at merimac springs st. park in mo(while trout fishing) there is a large vein there. i also found some near the gasconade river near fort.lenardwood