Job 41:24 in the World English Bible
His heart is as firm as a stone, yes, firm as the lower millstone.
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.