burstone

[bur-stohn] /ˈbɜrˌstoʊn/
noun
1.
any of various siliceous rocks used for millstones.
2.
a millstone of such material.
Also, buhrstone, burrstone.
Also called buhr, burr.
Origin
bur(r)4 + stone
British Dictionary definitions for burstone

buhrstone

/ˈbɜːˌstəʊn/
noun
1.
a hard tough rock containing silica, fossils, and cavities, formerly used as a grindstone
2.
a grindstone or millstone made of this rock
Word Origin
C18 burr, perhaps identical to burr1 (alluding to roughness)

burstone

/ˈbɜːˌstəʊn/
noun
1.
a variant spelling of buhrstone