Image: Coal Rank USGS
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Description: Diagram of coal ranking system used in the United States (US Geological Survey)
Title: Coal Rank USGS
Credit: "An introduction to coal quality", US Geological Survey, Professional Paper 1625-F
Author: Stanley P. Schweinfurth
Permission: Public domain, product of US federal government
Usage Terms: Public domain
License: Public domain
Attribution Required?: No
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