Millstone \Mill"stone`\, n. One of two circular stones used for grinding grain or other substance.

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

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.

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, or through, {a millstone, to see into or through a difficult matter. (Colloq.)

Source: Webster's Revised Unabridged Dictionary (1913)