Click the photo to see the larger version. During the Archean, the Earth still had its original atmosphere of nitrogen and carbon dioxide. That would be deadly for us but it was hospitable to many different microorganisms in the sea, including the first photosynthesizers. These organisms gave off oxygen as a waste product, which immediately bonded with the abundant dissolved iron to yield minerals like magnetite
. Today banded iron formation is our predominant source of iron ore. It also makes beautiful polished specimens
Learn more about the ancient origin of iron and about the Archean.
For more photos see the Sedimentary Rocks Gallery.