boulder vs concretion

boulder

noun
  • A large mass of stone detached from the surrounding land. 

  • A particle greater than 256 mm in diameter, following the Wentworth scale 

  • A large marble, in children's games. 

  • A session of bouldering; involvement in bouldering. 

verb
  • To engage in bouldering. 

concretion

noun
  • A rounded mass of a mineral, sometimes found in sedimentary rock or on the ocean floor. 

  • The action of making something concrete or the result of such an action. 

  • A solid, hard mass formed by a process of aggregation or coalescence. 

  • The process of aggregating or coalescing into a mass. 

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