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

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