Peacock Coal (CO-13)
Peacock coal is not a distinct variety. The coloring is a surface effect that usually does not last long after the coal is exposed. A thin film of material that refracts light causes the rainbow of color just as a thin film of oil on water causes iridescence. The film causing peacock colors is usually iron oxide deposited in fine cracks and pores in the coal by sulfur dioxide may also produce this effect.
Our Hoosier State Beneath Us: Coal