Click the photo to see the larger version. During the Proterozoic, 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 and hematite. 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 Proterozoic.
For more photos see the Sedimentary Rocks Gallery.
Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society