flagstone vs shale

flagstone

noun
  • A flat, rectangular piece of rock or stone used for paving or roofing. 

  • One of several types of rock easily split and suitable for making flagstones. 

shale

noun
  • A fine-grained sedimentary rock of a thin, laminated, and often friable, structure. 

  • A shell or husk; a cod or pod. 

verb
  • To take off the shell or coat of. 

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