October birthstone (4)

I believe the answer is:

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

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

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