A mill for grinding or pulverizing, in which a flat siliceous rock, generally of cellular quartz, rotates against a stationary stone of the same material. The Buhrstone mill is one of the oldest types of mill and, with either horizontal or vertical stones, has long been used to grind grains and hard materials. Grooves in the stones facilitate the movement of the material. Fineness of the product is controlled by the pressure between the stones and by the grinding speed. A finely ground product is achieved by slowly rotating the stone at a high pressure against the materials and its mate (see illus.). The capacity, or output, of a Buhrstone mill is low and its power requirements are high. The stones require frequent maintenance, even when grinding only slightly abrasive materials.