Job 41:24
His heart is as firm as a stone, yes, firm as the lower millstone.
Isaiah 48:4
Because I knew that you are obstinate, and your neck is an iron sinew, and your brow brass;
Jeremiah 5:3
O Yahweh, don't your eyes look on truth? You have stricken them, but they were 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
Yes, they made their hearts as hard as flint, lest they might hear the law, and the words which Yahweh of Armies had sent by his Spirit by the former prophets. Therefore great wrath came from Yahweh of Armies.