Albert Barnes Commentary Ezekiel 3:9

"As an adamant harder than flint have I made thy forehead: fear them not, neither be dismayed at their looks, though they are a rebellious house." — Ezekiel 3:9 (ASV)

Adamant - Or, diamond (Jeremiah 17:1), which was used to cut flint. Ezekiel’s firmness, being like that of a diamond, meant he was to penetrate the hardened hearts of a rebellious people. For “though” read “for.”