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 LA Times, The New York Times, The Wall Street Journal, the USA Today, The Times, The Washington Post, the King Feature Syndicate, The Mirror, the Newsday, The Star Tribune, the Universal and The Sunday Times.

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