EDIT: These are fusain and not "Coal Balls." They are concretions of fossilized charcoal wood, showing evidence of a forest fire.
Note from Jack: (copied from below)
These were collected in a Mazon Creek spoil pile from an older underground mine. I understand that the older underground mines would bring up everything from the seam of colchester coal and overlying shale. Then at the surface, they'd remove the "gob" -- non-coal stuff like these and add to the pile.