black as coal vs obsidian

black as coal

adj

  • Completely black. 

obsidian

adj

  • black 

noun

  • A type of naturally occurring black glass produced by volcanoes. 

  • A slightly bluish black, the color of obsidian glass. 

How often have the words black as coal and obsidian occurred in a corpus of books? (source: Google Ngram Viewer )