Banded Iron Formation
I just obtained this stunning piece of Banded Iron Formation (BIN), a rock from the bottom of an ancient sea, over 2.5 billion years old, which developed BEFORE the Earth had oxygen in its atmosphere or the sea.
The iron ore deposits began forming when the first organisms capable of photosynthesis began releasing oxygen into the waters. This oxygen immediately combined with the abundant dissolved iron (much which came out of hot thermal vents on the ocean floor) to produce these layers (over hundreds of millions of years).
BTW, this rock which was taken from a mine, is where most of the iron came from to make steel and everything made from iron and steel today.
I'll try to get some better photos up on this soon.