Adamant

Adamant renders the Hebrew shamir, the hardest known substance in the ancient world — most likely corundum, emery, or possibly diamond. The Greek word adamas from which the English term derives means unconquerable or indestructible. In the Old Testament, the adamant stone appears as a figure of unyielding hardness applied both to the human heart and to the prophetic calling. Ezekiel 3:9 records God telling the prophet that his forehead has been made harder than flint or adamant so that he would not shrink from declaring an unwelcome word to a rebellious house.

Zechariah 7:12 uses the same term for the stubborn heart of the people who refused to hear the law and the words of the former prophets — hearts made like an adamant stone in their resistance to God's word. Jeremiah 17:1 similarly uses a related term for the cutting point of iron and diamond used to engrave sin upon the tablets of the heart. Together these passages use the hardness of the adamant as a moral metaphor for obstinacy in the face of divine instruction.