To see into a millstone

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.
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   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]