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Banded Ironstone is an iron rich rock from another time in the earth's history. This sedimentary rock consists of layers of iron oxides and iron poor chert. Because iron oxidizes easily ironstone like this can no longer form. Why you may ask? Keep reading...
In Precambrian time, thousands of millions of years ago, the ocean water was rich in elemental iron but had very little free oxygen. At that time we would not have recognized the oceans of earth. They were not the beautiful blue that we have today but probable a green and they would have been poisonous to most living things of today.
Then along came the cyanobacteria! This type of bacteria was the first organism to use photosynthesis to make its food while producing oxygen as a byproduct. Cyanobacteria was wildly successful! Beginning about 2.7 billion years ago and continuing up to the Cambrian Period they filled the oceans all over the world producing mass quantities of oxygen. There are still cyanobacteria living today but not quite on the massive scale as in Precambrian time.
All of this oxygen combined with the iron already present in the water, changing the oceans from green to red. The oxidized iron sank to the bottom and was deposited in thick layers. Iron deposits like this could be several hundred meters thick and hundreds of kilometers across! This is the source of much of the world's iron resources today.
Ironstones cannot form today because there is only small amounts of elemental iron and of course oxygen is now present in the oceans and atmosphere thanks to our friends the cyanobacteria.
Banded ironstone makes up the lion's share of the earth's iron deposits and we use iron to make many thousands of things to make life easier. For rock collectors though, it also tells us the story of how our world became how it is today, and it can be as beautiful as it is interesting.
Some banded ironstone formations in Australia have red, gold, and black layers that polish well like the one pictured below.
You can get banded ironstone at fossilicious.com!