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The Collaborative International Dictionary of English v.0.48 (gcide)

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]