I thought I would share a stone I received from the Berea area, I was sent this stone as a natural bevel setter option. So far it is very hard and gives up particles that are extremely small. Cut speed reminds me of a much finer synthetic stone though I followed this stone with a slate stone that was heavily slurried to remove the imperfections that occur and are seemingly inherent in sandstone hones. Cheers.
As an aside if you rub it with a DMT and leave the grit on the stone it cuts rather alarmingly fast and coarsely.