6.5" Polished Tiger's Iron Section - South Africa

Tiger iron is composed mainly of tiger's eye, red jasper, and black hematite in an undulating banded pattern. It's mined primarily in two large deposits, one in South Africa and the other in Western Australia, both of which are over 2 billion years old.

While up for debate, there are many that believe tiger iron should technically be considered stromatolite, formed by ancient cyanobacteria over two billion years ago. One theory is tiger iron is a typical stromatolite that has undergone mineral replacement with iron oxide. The other is the microbes formed the banded iron directly while the stromatolite was being formed.

Even if tiger iron doesn't end up technically being a stromatolite, its formation along with the formation of other precambrian banded iron formations is indirectly due to the cyanobacteria that formed stromatolites. Oxygen was not present in the early atmosphere, but arose as a byproduct of photosynthesis by cyanobacteria. This oxygen combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layers on the ocean floor. The bands within the tiger iron would represent cyclical (seasonal?) variations in oxygen levels within Earth's oceans.

It is assumed that initially the Earth started with vast amounts of iron and nickel dissolved in the world's acidic seas. As photosynthetic organisms generated oxygen, the available iron in the Earth's oceans precipitated out as iron oxides. At a suspected tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced periods of free oxygen in the surface waters, alternating with periods of iron oxide deposition.

Tiger iron is gorgeous to look at and even more impressive when you consider it's evidence of life on earth several billion years ago.