Two heavy, round stones that are used to grind grain into flour.
Millstones are very old-fashioned and are used in traditional grain milling.
Examples:
The miller uses the millstones to make flour.
The big millstones crush the grain into flour.
The millstones are very heavy.
The millstones are like big, heavy wheels.
The millstones are as big as a small car.
The millstones turn and turn, making the flour.