Millstone, a hard and rough stone in one or many pieces, formed into cylindrical shape, from 3 to 7 ft. in diameter, and 8 to 18 in. thick, used together with another of the same size and shape for grinding grain, etc. The lower stone is firmly fixed in its bed, and is known as the "bedder." The upper one, called the "runner," is suspended over this so as to revolve with its lower face exactly parallel to the upper face of the lower stone, and more or less close to it according to the required fineness of the Hour. The grain is admitted through a hole in the centre of the upper stone from the hopper above; and as it is ground the flour escapes round the outer edges. Grooves are cut on the face of each stone, radiating from near the centre to the periphery, and One edge of these grooves is sharp and perpendicular to the face. The two stones being cut alike, when they are turned face to face these edges work against each other and crush the grain between them. The flat portions each side of the grooves are called "lands." The best millstones are made of buhrstone. (See Buhrstone.) They continue in use sometime as long as 20 years, the edges being occasionally recut.

Very hard granite is also used for millstones, and the Shawangunk sandstone has long been quarried at Esopus, N. Y., for the same purpose.