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

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