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buhr·stonealso burr·stone (bûr′stōn′)
A tough, silicified limestone formerly used to make millstones. It is typified by the presence of multiple cavities that originally housed fossilized shells.
1. (Geological Science) a hard tough rock containing silica, fossils, and cavities, formerly used as a grindstone
2. (Mechanical Engineering) a grindstone or millstone made of this rock
[C18 burr, perhaps identical to burr1 (alluding to roughness)]
any of various siliceous rocks used for millstones.