I am sure it is one of those antique oil stones. Extremely fine, relatively hard, came in typical coffin. But what is it? Color is 99% correct.
bigger size
http://traskrom.users.photofile.ru/p.../206423881.jpg
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http://traskrom.users.photofile.ru/p.../206423879.jpg
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http://traskrom.users.photofile.ru/p.../206423886.jpg