What kind of sharpening stone is it?

I received an old sharpening stone, from a relatives estate.  It seems to be an old stone, by the looks of it.  It is in a wooden carrier.  There are no visible markings on the stone.  

It would appear to be an oilstone, since it is apparent that oil was used on it.  It is a brown stone, although with the oil its mostly black, and seems to be a fairly fine grit.  It had been suggested I try to clean the stone off, in hopes of finding some markings somewhere.  This leads to the question, of what to clean it with.  Especially since I don't know what type of stone it is.  With my luck, I'd clean it like an oilstone, then find out its a waterstone, and I just ruined it.  Anyone have any suggestions?

Thanks

Dave