A type of porous volcanic rock used to make grinding stones or millstones.
From German 'Buhr' or French 'buhre,' referring to the volcanic stone quarried in the Rhine region. The term entered English through trade and milling industry vocabulary, becoming standard in mill-making terminology.
Buhr stone was revolutionary—it's porous enough to grind grain without generating dangerous heat, so millers preferred it over harder stones. Geography determined which regions became mill capitals because only certain areas had the right rocks.
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