icicle plant vs pebble plant

icicle plant

noun
  • A plant in the daisy family,Asteraceae, with long, powdery-white leaves (Helichrysum thianschanicum). 

pebble plant

noun
  • A lithops (succulent plant of genus Lithops) 

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