black coal vs firebrand

black coal

noun
  • coal 

  • high-quality coal 

firebrand

noun
  • A torch or other burning stick with a flame at one end. 

  • An argumentative troublemaker or revolutionary; one who agitates against the status quo. 

How often have the words black coal and firebrand occurred in a corpus of books? (source: Google Ngram Viewer )