Banded iron formation was laid down some 2.5 billion years ago during the Proterozoic Eon. It consists of black iron minerals and red-brown chert. (more below)
Photo by André Karwath from Wikimedia Commons
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.