Job 41:24 KJV
His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.
Job 41:24, King James Version
In context
23 The flakes of his flesh are joined together: they are firm in themselves; they cannot be moved.
24 His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.
25 When he raiseth up himself, the mighty are afraid: by reason of breakings they purify themselves.