'Millstone' in the Bible

“Do not take a pair of millstones or an upper millstone as security for a debt, because that is like taking a life as security.

But a woman threw the upper portion of a millstone on Abimelech’s head and fractured his skull.

At Thebez, who struck Abimelech son of Jerubbesheth? Didn’t a woman drop an upper millstone on him from the top of the wall so that he died? Why did you get so close to the wall?’—then say, ‘Your servant Uriah the Hittite is dead also.’”

His heart is as hard as a rock,as hard as a lower millstone!