'Iron' signifies natural truth, and consequently, the natural sense of the Word. At the same time, 'iron' also signifies the natural light of mankind. These two definitions together signify the power of truth.

In Revelation 9:9, it signifies something strong.

In Deuteronomy 8:9, 'iron' signifies natural or rational truth.

(References: Arcana Coelestia 425)