coal black vs onyx

coal black

adj
  • Of an intensely black colour, like that of coal. 

noun
  • A black colour, like that of coal; charcoal. 

onyx

adj
  • jet-black 

noun
  • A jet-black color, named after the gemstone. 

  • Any of various lycaenid butterflies of the genus Horaga. 

  • A banded variety of chalcedony, a cryptocrystalline form of quartz. 

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