Archaean Banded Iron from Wisconsin
"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 very labor-intensive job. The pictures comparing front and back reveals the value of this form of polishing.