'Adamant' in the Bible

As an adamant harder than flint have I made thy forehead. Fear them not, neither be dismayed at them, for they are a rebellious house.

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.