James Parker told me a long time ago that if you can slide a hammerstone down your jeans leg without it catching, it's too hard and slick. I've found that to be dead on. I can get good flakes with a relatively soft, gritty hammerstone, but the hard slick ones just cone the rock and take off short, thick flakes. The hammerstone he's using in the video looks a lot like the gritty, grainy, "rotten" granite/schist cobbles that I pick out of the rivers around here for hammerstones.