ferrous vs obdurate

ferrous

adj
  • Of or containing iron. 

  • Of compounds of iron in which it has a valence or oxidation number of 2. 

obdurate

adj
  • Hardened against feeling; hard-hearted. 

  • Stubbornly persistent, generally in wrongdoing; refusing to reform or repent. 

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