as hard

Isaiah 48:4
For I knew that you are stubborn; your neck is iron and your forehead is bronze.

Jeremiah 5:3
O LORD, do not Your eyes look for truth? You struck them, but they felt no pain. You finished them off, but they refused to accept discipline. They have made their faces harder than stone and refused to repent.

Zechariah 7:12
They made their hearts like flint and would not listen to the law and the words that the LORD of Hosts had sent by His Spirit through the earlier prophets. Therefore great anger came from the LORD of Hosts.