An extremely hard substance.
But the house of Israel will not hearken unto thee; for they will not hearken unto me: for all the house of Israel are impudent and hardhearted. Behold, I have made thy face strong against their faces, and thy forehead strong against their foreheads. As an ADAMANT harder than flint have I made thy forehead: fear them not, neither be dismayed at their looks, though they be a rebellious house. ( Ezekiel 3:7-9 )