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Your better pictures and the additional data proofs that I was wrong.

Best regards,
Bart.

Still sounds like that Coticule to me.
The speed you report, reasonably fast discoloring of slurry while honing on it. Scratch pattern looking hazy (as it it was micro-sandblasted?). It all sounds...

It looks like an oxidized "La Grosse Blanche" Coticule.

I'm far from sure. I don't know a lot about any other natural hones than Coticules.

Please compare to this picture:
(a heavily oxidized...