As an adamant harder than flint have I made thy forehead,.... Or, "than a rock" ; the "adamant" has its name in Greek, because it cannot be conquered or subdued, neither by the hammer, nor by fire; the one cannot break, nor this other consume it; land it is called "shamir" in Hebrew, from its preserving itself from both; it will cut iron in pieces, which is harder than stone, and therefore must be harder than that.
Bochart takes it to be the same with "smiris", a hard stone, which jewellers use to polish their gems with; see Jer 17:1. The design of the simile is to set forth the courage and fortitude of mind the prophet was endowed with, in order to face an impudent and hardhearted people;fear them not, neither be dismayed at their looks, though they [be] a rebellious house; See Gill on "Eze 2:6".