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that would be a good guess if its a hard stone. you will know when your lapping one, it takes awhile. mine is kind of brown, tan

here are also a link

http://straightrazorpalace.com/hones/55863-how-do-i-determine-grit-unmarked-unknown-stone.html

i usually put a razor to it at different points of the process and try to find the slot it fits in with the known stones i have. set the bevel with a 1k, then go to the mystery stone. does it improve...