I'm very interested in exploring this question. I need to get together with someone who has a rock saw to cut these slabs into useable hones. the hornstone and quartz will undoubtedly work as fine finishers, and the fort payne will be useful as a coarser hone, probably in the 1000 grit range. flint Ridge is very glass like, but might be a finisher, I don't know. Obsidian is glass, so it would not be useful... too bad, as I have several hone like slabs already cut and chamfered! the hornstone is very interesting, though, and I'm convinced it would be very useful as a finishing hone. I would be willing to send a piece of hornstone to one of the honemeisters to evaluate; some of the other materials also, just to get an expert opinion.