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

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