Source : Webster's Revised Unabridged Dictionary (1913)

Millstone \Mill"stone`\, n.
   One of two circular stones used for grinding grain or other
   substance.

         No man shall take the nether or the upper millstone to
         pledge.                                  --Deut. xxiv.
                                                  6.

   Note: The cellular siliceous rock called buhrstone is usually
         employed for millstones; also, some kinds of lava, as
         that Niedermendig, or other firm rock with rough
         texture. The surface of a millstone has usually a
         series of radial grooves in which the powdered material
         collects.

   {Millstone girt} (Geol.), a hard and coarse, gritty
      sandstone, dividing the Carboniferous from the
      Subcarboniferous strata. See {Farewell rock}, under
      {Farewell}, a., and Chart of {Geology}.

   {To see} {into, or through, {a millstone}, to see into or
      through a difficult matter. (Colloq.)