I've tried the slate and it was a waste of time. YMMV. I also tried a suzuri used in making ink for Japanese calligraphy. The one I have is from a mountain in Tokyo IIRC ........ no good action there either. I am going to go rock hounding though. Living in South FL all I've come up with is sea shells. So far none of them make a good slurry but I'm going to keep trying ......