Three years ago I blogged here about meeting Japan's only full-time miner and maker of natural waterstones. Without repeating what I wrote, here is the link:
A week ago I was walking my dog in the park below the falls in my town of Vergennes, Vermont. The park where we walked is visible to the left of the falls. My dog ran up the hillside and following him I noticed the entire hillside was covered with hundreds of stone offcuts. Each had natural irregular faces and one sawn face. The stone is a kind of green slate, I think. Obviously, these are all offcuts of some operation making finished stone, but what struck me was how flat the sawn faces were. I wondered if the stone might be good for sharpening, because I remember in talking to the Japanese miner, struggling to translate the Japanese name for the type of stone he uses, and our online dictionary coming up with the English terms "tuff" and "slate."
My block was irregular so I wanted to saw off one part.
I noticed a natural split and took a chance and it broke easily and got me nearly what I wanted.
I trimmed the rest with my saw using a $5 composition masonry blade. It cut easily, so I had a suspicion it would make a good sharpening stone.
Here is my block with the original sawn face up.
Using some water and my #150 grit diamond stone I was able to very quickly flatten it.
It looks like a very nice surface.
I did some sharpening on one of my plane blades and got a nice, mirrored finish. The stone seems soft enough to give away but still make a polished edge. I don't see any sign that it wants to fracture more. At over three inches thick I think I should get my money's worth out of it.
