'Millstone' in the Bible

No man shall take the mill or the upper millstone to pledge; for he takes [a man's] life to pledge.

A certain woman cast an upper millstone on Abimelech's head, and broke his skull.

who struck Abimelech the son of Jerubbesheth? Didn't a woman cast an upper millstone on him from the wall, so that he died at Thebez? Why did you go so near the wall?' then you shall say, 'Your servant Uriah the Hittite is dead also.'"

His heart is as firm as a stone, yes, firm as the lower millstone.