Job 41:24

“His heart is as hard as a stone,
Even as hard as a lower millstone.

Isaiah 48:4

“Because I know that you are obstinate,
And your neck is an iron sinew
And your forehead bronze,

Jeremiah 5:3

O Lord, do not Your eyes look for truth?
You have smitten them,
But they did not weaken;
You have consumed them,
But they refused to take correction.
They have made their faces harder than rock;
They have refused to repent.

Zechariah 7:12

They made their hearts like flint so that they could not hear the law and the words which the Lord of hosts had sent by His Spirit through the former prophets; therefore great wrath came from the Lord of hosts.

Treasury of Scripture Knowledge

Summary

His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.

Bible References

As hard

Isaiah 48:4
“Because I know that you are obstinate,
And your neck is an iron sinew
And your forehead bronze,
Jeremiah 5:3
O Lord, do not Your eyes look for truth?
You have smitten them,
But they did not weaken;
You have consumed them,
But they refused to take correction.
They have made their faces harder than rock;
They have refused to repent.
Zechariah 7:12
They made their hearts like flint so that they could not hear the law and the words which the Lord of hosts had sent by His Spirit through the former prophets; therefore great wrath came from the Lord of hosts.