magnum opus vs piece de resistance

magnum opus

noun
  • A great or important work of literature, music or art, a masterpiece. 

  • The best, most popular, or most renowned achievement of an author or artist, representing their major life effort. 

  • The process of working with the prima materia to create the philosopher's stone. 

piece de resistance

How often have the words magnum opus and piece de resistance occurred in a corpus of books? (source: Google Ngram Viewer )