Job 41:24
His heart is firm as a stone, yea, firm as the nether millstone.
Isaiah 48:4
Because I knew that thou art obstinate, and thy neck is an iron sinew, and thy brow brass,
Jeremiah 5:3
Jehovah, are not thine eyes upon fidelity? Thou hast smitten them, but they are not sore; thou hast consumed them, they have refused to receive correction: they have made their faces harder than a rock; they have refused to return.
Zechariah 7:12
And they made their heart as an adamant, that they should not hear the law, and the words that Jehovah of hosts sent by his Spirit by the hand of the former prophets: therefore was there great wrath from Jehovah of hosts.