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