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You know about this thread right? https://sharprazorpalace.com/hones/107276-i-found-over-there.html

Hard to tell from the pictures, but I'm thinking schist. I tried a few pieces of schist and my results were not worth a ...., but I have heard of schist being used for hones so some of it must be OK....

Once you get a big enough patch lapped flat you can do some testing to see if it's even worth going further. I've learned to pick up some small pieces of given layers for testing, (I know that you...