Job 41:24

²² Strength resides in his neck, and dismay leaps before him.

²³ The folds of his flesh are tightly joined; they are firm and immovable.

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

²⁵ When Leviathan rises up, the mighty are terrified; they withdraw before his thrashing.

²⁶ The sword that reaches him has no effect, nor does the spear or dart or arrow.

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
For I knew that you are stubborn; your neck is iron and your forehead is bronze.
Read in context
Jeremiah 5:3
O LORD, do not Your eyes look for truth? You struck them, but they felt no pain. You finished them off, but they refused to accept discipline. They have made their faces harder than stone and refused to repent.
Read in context
Zechariah 7:12
They made their hearts like flint and would not listen to the law or to the words that the LORD of Hosts had sent by His Spirit through the earlier prophets. Therefore great anger came from the LORD of Hosts.
Read in context