hallmark vs watermark

hallmark

verb
  • To provide or stamp with a hallmark. 

noun
  • An official marking made by a trusted party, usually an assay office, on items made of precious metals. 

  • A distinguishing characteristic. 

watermark

verb
  • To mark paper with a watermark. 

  • To mark a datafile with a digital watermark. 

noun
  • A mark indicating the level to which water has risen. 

  • A translucent design impressed on the surface of paper and visible when the paper is held to the light. 

  • A logo superimposed on a digital image, a television broadcast, etc. 

  • A value stored in a datafile to ensure its integrity, so that if the file's contents are changed then the watermark will no longer match the contents. 

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