Banded iron formations consist of alternating layers of iron oxides and other minerals. This specimen is a variety called zebra jasper or banded jasper.
As with most geological formations these are extremely old, and likely formed around 2.5 billion years ago. This time period coincides with the Great Oxidation Event in Earth’s history.
During this time on Earth, the atmosphere and shallow oceans underwent an increase in oxygen primarily produced by cyanobacteria. As the oxygen increased, the atmosphere and oceans changed from chemically reducing to oxidizing.
The oceans covered much more of the Earth’s surface at the time and their soluble iron content began to oxidize and settle to the ocean floor to form these deposits. Now you’ve got a piece of ancient sea floor.