Either of two flat, round stones used for grinding grain to make flour.
The stationary bottom stone is carved with shallow grooved channels that radiate from the centre. The upper stone rotates horizontally, and has a central hole through which grain is poured. The channels of the bottom stone lead the grain onto the flat grinding section, called the land, and to the edge, where it emerges as flour. The best millstones are made from French buhrstone, quarried near Paris. In the U.S., quartz conglomerate, quartzite, sandstone, or granite is used. Stone-ground flour accounts for only a small proportion of milled flour today.