Job 41:24 in the King James Version
His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.
Context
23 The flakes of his flesh are joined together. They are firm on him. They can’t be moved.
24 His heart is as firm as a stone, yes, firm as the lower millstone.
25 When he raises himself up, the mighty are afraid. They retreat before his thrashing.