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. Made their hearts as an adamant stone - שמיר shamir may mean the granite. This is the hardest stone with which the common people could be acquainted. Perhaps the corundum, of which emery is a species, may be intended. Bochart thinks it means a stone used in polishing others. The same name, in Hebrew, applies to different stones.
Related Commentaries of Zechariah 7:12
Zechariah 7:12
12 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 byc the former prophets: therefore came a great wrath from the LORD of hosts.