Adamant. The Hebrew word shemir is used in Jeremiah 17:1 (where the Authorized Version gives "diamond" for a stone used in engraving on gems. In Zechariah 7:12 it appears, as it does here, as a type of exceeding hardness.
It is not found elsewhere in the Old Testament. It is commonly identified with the stone known as corundum, which appears in some of its forms as the sapphire and the Oriental ruby, and also as the stone the powder of which is used as emery.
The special point of the comparison is, of course, that the adamant was actually used to cut either flint itself or stones as hard as flint. Neither be dismayed at their looks. The words indicate the extreme sensitiveness of the prophet's natural temperament.
He had shrunk not only from the threats and revilings of the rebellious house, but even from their scowls of hatred.