Iridescent birthstone (4)

I believe the answer is:

opal

'iridescent 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 "often semi-transparent" , "Iridescent quartz-like stone" , "Something milky-white" , "Translucent gemstone" , "Shimmering stone" .)

I've seen this clue in the New York Times, the Wall Street Journal and the USA Today.

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