Coal is not a mineral, because it can not be expressed as a chemical formula, and therefore, does not have a definate crystaline structure. Coal is predominately carbon. The specimen in the photo is Bituminous Coal, with a carbon content of about 90%, and a BTU rating of over 13,000 BTU/pound. Even though it is predominately carbon the other constituents of coal can be almost anything that was in the swamp when it was formed. I put it in the mineral section because numerous questions have been forwarded to this site asking why coal is not a mineral. The carbon content of coal ranges from 40% for low ranked coal (lignite) to about 98% for Anthracite Coal.

The specemin in the photo is from the East Kentucky Coal Fields.