Paleoproterozoic Banded Iron from Michigan
"The Rusting of the Earth"

This banded iron is metasediment (sediment that has undergone metamorphism) and has a high content of magnetite (Fe3O4), a natural magnet, and yielding the midnight black coloration; minute crystalline forms of other mineral-based compounds are also present. Whether the sediment was of bioorganic origin (stromatolite) is indeterminable, but likely. The specimen has been hand-polished to a glassine finish on one side with fine carbide and diamond abrasive, a labor-intensive job.

The polished side's reflected colors highly depends on the angle of reflected light. Depending on angle, colors vary from black to gray and red to silver. This is a very pretty example of banded iron that encodes a defining event in geological history and evolution, i.e., the rusting of the earth due to bacterial-produced oxygen that also led to the creation of atmospheric oxygen. At the time this banded iron was formed, the earth had been rusting for about 1,500 million years, and about another 500 million years would be required before the process exhausted the marine iron supply.