Heavy Metal (but no iron)

Parsha Curiosities: Parshat Terumah Cracking the Code on mysterious themes & ideas in the weekly Parsha! ( ) After being found everywhere, but dwelling nowhere specific for 2249 years, G-d has informed the Jewish people that he now intends to dwell amongst them. He'd like them to craft a mobile home so that he can move along with them through the Sinai desert. He conveys exact instructions, and even tells them precisely what materials to use –including some “heavy metals” like gold, silver and copper. Tellingly, iron is absent. Why might that be, and if avoiding cheaper utilitarian substances, why not just “go with the gold” – maintaining only highest of standards?!