'Adamant' in the Bible

"I have made your face adamant to match their faces, and your forehead hard to match their foreheads.

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

Yea, they made their hearts as an adamant stone, lest they should hear the law, and the words which the LORD of hosts hath sent in his spirit by the former prophets: therefore came a great wrath from the LORD of hosts.