jet-black vs onyx

jet-black

adj

  • Pitch-black; of the blackest black. 

noun

  • Coal-black; the blackest black. 

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 jet-black and onyx occurred in a corpus of books? (source: Google Ngram Viewer )