Millstone - Wikipedia
Millstones or mill stones are stones used in gristmills, for grinding wheat or other grains. They are sometimes referred to as grindstones or grinding stones.. Millstones come in pairs: a convex stationary base known as the bedstone and a concave runner stone that rotates. The movement of the runner on top of the bedstone creates a "scissoring" action that grinds grain trapped between the stones.