Adamant: A Flint
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Ezekiel 3:9
I will make your forehead like a diamond, harder than flint. Do not be afraid of them or dismayed at their presence, even though they are a rebellious house.”
Zechariah 7:12
They made their hearts like flint and would not listen to the law or to 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.