Coal is a carbon-rich, biogenic sedimentary rock.  Many coal ranks exist, such as lignite coal, sub-bituminous coal, and bituminous coal.  Other varieties include cannel coal, semi-cannel coal, bone coal, and stone coal.  Seen here is anthracite coal, which is metamorphic variety, the result of very low grade metamorphism ("anchimetamorphism") of ordinary coal.  The iridescent coating makes the coal quite colorful, resulting in the term "peacock coal".  I have yet to see specific, convincing information about the identity of iridescent coatings on peacock coal, but I strongly suspect it's turgite (= hydrous iron oxide).
Provenance: unrecorded/undisclosed (purchased from a gift shop at the Pioneer Tunnel Coal Mine in Ashland, Pennsylvania, USA)
James St. John
CC BY 2.0
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