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Webster 1913 Edition

Noun.

[OE.

bur

a whetstone for scythes.]

(Min.)

A cellular, flinty rock, used for mill stones.

[Written also

burrstone

Noun.

A subspecies of silex or quartz, occurring in amorphous masses, partly compact, but containing many irregular cavities. It is used for mill-stones.

This word is often written burr-stone.

buhrstone ‎(plural buhrstones)