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

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