cast iron vs ironstone

cast iron

noun
  • A hard and brittle, but strong, alloy of iron, carbon, and silicon, formed by casting in a mould. 

adj
  • Inflexible or without exception. 

  • Made of cast iron. 

  • Durable; tough; resilient. 

ironstone

noun
  • Any ore of iron which is impure through the admixture of silica or clay. 

  • A type of vitreous pottery similar to stoneware 

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