Job 41:24
His heart is as firm as a stone; yea, as hard as a piece of the lower millstone.
Isaiah 48:4
Because I knew that you are obstinate, and your neck is an iron sinew, and your brow bronze;
Jeremiah 5:3
O LORD, are not your eyes upon the truth? you have stricken them, but they have not grieved; you have consumed them, but they have refused to receive correction: they have made their faces harder than a rock; they have refused to return.
Zechariah 7:12
Yea, they made their hearts as a flint stone, lest they should hear the law, and the words which the LORD of hosts has sent by his Spirit through the former prophets: therefore came a great wrath from the LORD of hosts.