Tryed, tested and true :) i have a stone from one of my grandads too, i have nooooo idea what it is and i havent even seen it since i was in my teens (around 20 years ago) i just know now from memory that it was very well heavily used for tool sharpening, it was almost grining, and recalling the texture and apperance, it was some sort of hard, extremely dark yellow natural stone, almost like a nicotene yellow, im gonna have to locate that.