crack rock vs freebase

crack rock

noun
  • crack cocaine 

freebase

noun
  • The purified, dry form of certain illegal drugs, especially cocaine. 

  • The purified, dry form of an amine, especially an alkaloid natural product, that is normally used in solution. 

verb
  • To purify a drug by crystallization. 

  • To use a purified drug, especially cocaine, by heating it and inhaling the fumes produced. 

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