I was pawing through an old toolbox that was given to me from an old GF's grandfather. he had been a surgeon in upstate NY around the turn of the last century. it seems likely that he used it as a hone for his razors and possibly his medical instruments. it came to me broken on the one end. that was back in 1978. it was covered with black gunk and I have been ignoring it for all this time. I soaked in in SG for a day and here is what I found. the stone is very hard and not porous. any guesses on what it is?