What is "burrstone"

Buhrstone \Buhr"stone`\, n. [OE. bur a whetstone for scythes.] (Min.) A cellular, flinty rock, used for mill stones. [Written also burrstone.]

alt. A tough, silicify limestone formerly used to make millstones. It is typified by the presence of multiple cavities that originally housed fossilized shells. n. A tough, silicify limestone formerly used to make millstones. It is typified by the presence of multiple cavities that originally housed fossilized shells.