The sea stacks

I am thinking of going with round river rocks for the sea stacks. Found a nice bag of large ones. How am I going to get the smooth smaller ones I need? I am going to break bigger rocks up and then tumble them down to size. It will take a few months but on a board that will take a year it seems within reason. I am also going to have to grind the surface of the river rocks down so they sit flat on the ocean and look realistic.