Millstone
Millstone , noun
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.
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.
Collocations (2)
Millstone girt (Geology) , 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. [Colloquial]