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Take a look at the site of Belgischer Brocken. Some but not all of their info is in English.

The Thuringian razor hones I sell on ebay are uniform grey with small pyrite inclusions. Feedback from buyers suggests that grit wise the Thuringians are somewhere between coticules on the lower end...

Excellent research, Yannis!!

If you have both you can check under the microscope.

This is the info by Belgischer Brocken: http://www.belgischerbrocken.com/

They say the blues and yellow are identical with exception or...

The Belgian blue is used by many on Nassrasur.com (German site) as a finishing hone. It is said to be only 6K although I never heard the garnets are bigger in the blue compared to the coticule.
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