or buhr·stone, burr·stone
any of various siliceous rocks used for millstones.
a millstone of such material.
Origin of burstone
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
burstone or burrstone
a hard tough rock containing silica, fossils, and cavities, formerly used as a grindstone
a grindstone or millstone made of this rock
Word Origin for buhrstone
C18 burr, perhaps identical to burr 1 (alluding to roughness)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012