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a round, dressed natural or artificial stone; the working part of a set of millstones, which hulls glumaceous crops and mills grain and other materials. Millstones working in pairs were widespread in the third and fourth centuries B.C.; prior to that grain was ground in mortars. Millstones were originally turned by humans or animals, but when they became larger in size they came to be powered by water and wind.
Millstones are made of one or several pieces (natural) or small particles (artificial) of rock. Hard types of rock such as quartzite (the best), sandstone, and granite are used for natural millstones; artificial millstones are made of crushed flint, quartzite, and emery or a mixture of emery and flint, cemented together by magnesium chloride or caustic magne-site. On their working surfaces millstones have grooves, the edges of which are the cutting parts. Millstones with sharp edges split grain particles more rapidly.
A. IA. SOKOLOV