'Iron' in the Bible

“I will go before you and level the mountains;I will shatter the doors of bronze and cut through the bars of iron.

“Because I know that you are obstinate,And your neck is an iron tendonAnd your brow is bronze [both unyielding],

“Instead of bronze I will bring gold,And instead of iron I will bring silver,And instead of wood, bronze,And instead of stones, iron.And [instead of the tyranny of the present] I will appoint peace as your officers,And righteousness your rulers.