'Adamant' in the Bible

As an adamant harder than a rock I have made thy forehead; thou dost not fear them, nor art thou affrighted before them, for a rebellious house are they.'

And their heart they have made adamant, Against hearing the law, and the words, That Jehovah of Hosts sent by His Spirit, By the hand of the former prophets, And their is great wrath from Jehovah of Hosts.