Zakharia 7:12

7:12 Indeed, they made their heart as hard as diamond, ¹  so that they could not obey the Torah and the other words the Lord who rules over all had sent by his Spirit through the former prophets. Therefore, the Lord who rules over all had poured out great wrath.

[7:12]  ¹ tn The Hebrew term שָׁמִיר (shamir) means literally “hardness” and since it is said in Ezek 3:9 to be harder than flint, many scholars suggest that it refers to diamond. It is unlikely that diamond was known to ancient Israel, however, so probably a hard stone like emery or corundum is in view. The translation nevertheless uses “diamond” because in modern times it has become proverbial for its hardness. A number of English versions use “flint” here (e.g., NASB, NIV).