The Collaborative International Dictionary of English v.0.48:

Millstone \Mill"stone`\, n.
   One of two circular stones used for grinding grain or other
   substance in a mill[1].
   [1913 Webster]

         No man shall take the nether or the upper millstone to
         pledge.                                  --Deut. xxiv.
                                                  6.
   [1913 Webster]

   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.
         [1913 Webster]

   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 a millstone or To see through a millstone, to
      see into or through a difficult matter. (Colloq.)
      [1913 Webster]