Volcanic SCPs are different — much lighter in color and easy to distinguish from the dark black spheres formed by high - temperature fossil - fuel burning in coal - fired power plants and vehicles. (scientificamerican.com)
The more coal and oil burned, the more black spheres formed, making such soot the perfect record of a swelling fossil - fuel pyromania. (scientificamerican.com)
Some of that carbon remains bonded tightly to its fellow carbons, unburned, forming a little black sphere. (scientificamerican.com)