building block vs flagstone

building block

noun
  • A component that is part of a larger construction, such as a brick or a stone. 

  • A block made of wood or plastic that is used as a children's toy and can be stacked or joined to form a larger structure. 

  • A component that is part of a larger system. 

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. 

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