Job 41:24

²² In his neck remaineth strength, and sorrow is turned into joy before him.

²³ The flakes of his flesh are joined together: they are firm in themselves; they cannot be moved.

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.

²⁶ The sword of him that layeth at him cannot hold: the spear, the dart, nor the habergeon.

Do read the surrounding verses too, because the context often says more than a single verse.

Compare

BSB: His chest is as hard as a rock, as hard as a lower millstone!
KJV: His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.
WEB: His heart is as firm as a stone, yes, firm as the lower millstone.
ASV: His heart is as firm as a stone; Yea, firm as the nether millstone.
DARBY: His heart is firm as a stone, yea, firm as the nether [millstone].
BLB: His heart is as firm as a stone, even as firm as a lower millstone.

ÅB: Dens hjerte er fast som stein, fast som den underste kvernsten.
OTB: Hans hjerte er hardt som en stein,
Ja, så hardt som en nedre kvernstein.
B1930: Dens hjerte er fast som sten, fast som den underste kvernsten.

Isaiah 48:4
Because I knew that thou art obstinate, and thy neck is an iron sinew, and thy brow brass;
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Jeremiah 5:3
O LORD, are not thine eyes upon the truth? thou hast stricken them, but they have not grieved; thou hast consumed them, but they have refused to receive correction: they have made their faces harder than a rock; they have refused to return.
Read in context
Zechariah 7:12
Yea, they made their hearts as an adamant stone, lest they should hear the law, and the words which the LORD of hosts hath sent in his spirit by the former prophets: therefore came a great wrath from the LORD of hosts.
Read in context