Banded Iron Formation from Jasper Knob in the UP of Michigan

This is still my favorite rock of all time. My husband and I collected this specimen in the mid-1980's on a field trip with our geology professor Dr. Palmquist. When we got back to the lab, my husband ground it down on two sides to see the banding. The photos do not do this rock justice.
The following facts are from the about.com geology guy.
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. 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.