Job 41:24
"His heart is as firm as a stone; yea, as hard as a piece of the nether millstone."
Cross References (ranked by relevance)
Isaiah 48:4
Because I knew that thou art obstinate, and thy neck is an iron sinew, and thy brow brass;
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.
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.