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I would clean, I've found a few natural stones that probably laid in the bottom of tool boxes for 50 years and the user possibly used old motor oil as a honing fluid!! Repeated Easy-Off and boilings...

Ok, that's a good idea??

Another thing to try is, go to the good-will and buy a cheap pan, fill it 1/2 with water and put in some Dawn dish soap, boil the stone for an hour or so, you'll be surprised how much junk will boil...