shale vs tortoise shell

shale

noun
  • A shell or husk; a cod or pod. 

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

verb
  • To take off the shell or coat of. 

tortoise shell

noun
  • The shell of a tortoise, sometimes inaccurately the shell of a turtle, or that material. 

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