Charles Ellicott Commentary Zechariah 7:12

"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." — Zechariah 7:12 (ASV)

Adamant stone means a very hard stone; “diamond” is the modern form of the word. “Adamant,” adhámas, meaning in Greek unconquerable, was originally applied to “steel” (Hesiod). The Septuagint explains the metaphor, made the heart disobedient.