'Adamant' in the Bible

As an adamant harder than flint have I made thy forehead: fear them not, neither be dismayed at their looks, though they are a rebellious house.

Yea, they made their hearts as an adamant stone, lest they should hear the law, and the words which Jehovah of hosts had sent by his Spirit by the former prophets: therefore there came great wrath from Jehovah of hosts.