nacreous vs opaline

nacreous

adj
  • Of, or resembling nacre (mother of pearl). 

  • Exhibiting lustrous or rainbow-like colors. 

opaline

noun
  • any of several minerals resembling opal, including a variety of yellow chalcedony 

  • a clear to white liquid secreted by sea hares (genus Aplysia) that becomes viscous upon contact with water 

  • opal glass 

adj
  • resembling opal in its iridescence; having a milky white iridescence. 

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