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

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