Though they may seem rock solid, the ancient sedimentary rocks called iron formations – the world’s chief economic source of iron ore – were once dissolved in seawater. How did that iron go from a dissolved state to banded iron formations? Dr. Itay Halevy and his group in the Weizmann Institute of Science’s Earth and Planetary Sciences Department suggest that billions of years ago, the “rust” that formed in the seawater and sank to the ocean bed was green – an iron-based mineral that is rare on Earth today but might once have been relatively common.