limestone vs plaster of Paris

limestone

noun
  • Pertaining to or made of limestone. 

  • An abundant rock of marine and fresh-water sediments; primarily composed of calcite (CaCO₃); it occurs in a variety of forms, both crystalline and amorphous. 

plaster of Paris

noun
  • A hemihydrate of calcium sulfate, made by calcining gypsum, that hardens when moistened and allowed to dry; used to make casts, molds and sculpture. 

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