I find I get the best results with my ABO tools on obsidian and glass. I prefer the ulna for pressure flaking and antler for percussion. As the stone's hardness increases, I tend to increase the hardness of my tools. When I work rhyolite, I usually end up with a small ball peen hammer after getting frustrated when I can't get a flake with anything else. I may end up making a little hammer-like bopper from a small hard hammer stone and trying that.