bur vs grindstone

bur

noun
  • A rotary cutting implement having a selection of variously shaped heads. 

  • Any of several plants having such husks. 

  • A rough, prickly husk around the seeds or fruit of some plants. 

grindstone

noun
  • An abrasive wheel for sharpening, polishing or grinding. 

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