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Whatever it is. It gives a lovely and sharp edge. Tend to use it purely with water after coti slurry on bbw then bbw slurry diluting down on bbw.

Oops. Doubled images.

Anyone with knowledge of coticules willing to hazard a guess as to vein of this stone.

Feels and sounds quite gritty when honing. Seems to self slurry very slightly, but no visible swarf so far....