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.
(Science: geology) Millstone girt, a hard and coarse, gritty sandstone, dividing the Carboniferous from the subcarboniferous strata. See farewell rock, under Farewell, and chart of geology. To see into, or through, a millstone, to see into or through a difficult matter.