This specimen is thought to be over 2.5 billion years old. Bacteria and algae appeared at this time in the water. Volcanic activity produced increased atmospheric pressure and temperature. The higher temperature caused serpentization of the sediments thus the green color. In between the layers the micro stromatolites (like algal mats).
This fossil was found in southern Africa's Greenstone Belt. Its green color definitely matches the formation name. Unique about the fossil material is it is called butterstone dated to the geologic eon Archean.
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In addition, here are pictures of oolitic agate formed on a Wyoming seafloor during the Eocene. Check it out on eBay: CLICK HERE