'Millstone' in the Bible

No man shall take the nether or the upper millstone to pledge: for he taketh a man's life to pledge.

And a certain woman cast a piece of a millstone upon Abimelech's head, and all to brake his skull.

Who smote Abimelech the son of Jerubbesheth? did not a woman cast a piece of a millstone upon him from the wall, that he died in Thebez? why went ye nigh the wall? then say thou, Thy servant Uriah the Hittite is dead also.

His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.