I got this stone as a gift from a friend in England. He could not tell me anything about it other than he found it in his Grand father's tool shed when they where cleaning the place up (his Grand Father had recently passed). It is a pretty hard stone with an interesting coloration of grayish - blue - green ranging to a sort of pinkish - red, I tried it with some water and it produced a white slurry that rapidly turned gray / black when I sharpened a knife on it. It seems to be a pretty fast cutting stone. Could anyone provide any more information on what this stone might be?