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"SOmeone should really write up an article on the history of different natural hones, and the geology and material science behind them. (I'm an academic, so that's where my mind goes.) "

I...

I would think that the characteristics if a good finishing stone would be one that has a composition of very fine grained(if not microcrystaline) material which is harder than steel, but with a...

What we have here is a Chlorite Schist. The green color and the metamorphic texture is a dead givaway. You could call it a Phyllitic Chlorite Schist if it has a sheen to it in the light.

Now,...