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The fact that most labels call them "special" doesn't make them more special than any other kind of stone. And, they are the "European" kind, slate like, not like novaculite or the "unique" Jnat type...

It's a German razor hone that was exported to France, as simple as that. And, since it advises you to use it with water, it could be a Thuringian.
Now, about the color of the slurry, a Thuringian's...