Olympic Australis is a noted one (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Milky-white mineral" , "Rock" , "Many-coloured gem" , "Gem stone" , "Gem with delicate colours" .)

I've seen this clue in The New York Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.