John Gill 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)

Yea, they made their hearts [as] an adamant stone
The word here used is translated a "diamond" in (Jeremiah 17:1) and it is said to be harder than a flint, (Ezekiel 3:9) . The Jewish writers say ^F7

and as hard is naturally the heart of man, and which becomes more so by sinning, and obstinate persisting in it, that nothing can remove the hardness of it but the powerful and efficacious grace of God: as hard as the adamant is, it is to be softened by the blood of a goat, as naturalists say ^F8 ; so the blood of Christ sprinkled on the heart, and a sense of forgiveness of sin by it, will soften the hardest heart:

lest they should hear the law, and the words which the Lord of hosts has sent in his Spirit by the former prophets ; the words of reproof, admonition, caution, and exhortation, which Jeremiah and others were sent to deliver to them, under the influence of the Spirit of God:

therefore came a great wrath from the Lord of hosts ; which brought the Chaldeans upon them, who carried them captive into Babylon.

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FOOTNOTES:

• F7: it is a worm like a barley corn, so strong as to cut the hardest stones in pieces; Moses (they say) used it in hewing the stones for the two tables of the law, and in fitting the precious stones in the ephod; and Solomon in cutting the stones for the building of the temple; and is so hard that it cannot be broken by iron:
• F8: as naturalists say