October birthstone (4)

I believe the answer is:

'october birthstone' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for opal that I've seen before include "Something milky-white" , "Translucent gem" , "Iridescent gemstone" , "Variegated gemstone" , "Milky-white gemstone with flashes of colour" .)

I've seen this clue in The New York Times, the Universal, The Sunday Times, the Newsday, The Wall Street Journal, The Mirror, The Times, the LA Times, the King Feature Syndicate, The Star Tribune, the USA Today and The Washington Post.

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