The Collaborative International Dictionary of English v.0.48:

Buhrstone \Buhr"stone`\, n. [OE. bur a whetstone for scythes.]
   (Min.)
   A cellular, flinty rock, used for mill stones. [Written also
   burrstone.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Burrstone \Burr"stone`\, n.
   See Buhrstone.
   [1913 Webster]