art object vs magnum opus

art object

noun
  • Synonym of objet d'art (“artwork”) 

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. 

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