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Slate. I think someone said the base of my Coti was slate so I'll have to take a second look at it and see how it can be utilized.
The black stone was pretty flat on top except for the scratches,...

Well, if you don't know how to classify or test the stone (fingernail test was a good tip), a name can be very helpful to someone less knowledgeable about stones and honing. I have a couple stones...

Saw these at a shop, but I didn't see anything familiar about them. The light colored stone must have been softer because it had a good groove worn into it.
The black stone just had scratches and...