Millstone /(?)/

Mill·stone

Millstone

n.
  1. One of two circular stones used for grinding grain or other substance in a mill{1}.
    No man shall take the nether or the upper millstone to pledge.
    — Deut. xxiv. 6.

Phrases & Compounds
Millstone girt
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
to see into or through a difficult matter.