Iridescent gemstone mined extensively in Australia (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Semiprecious stone of white and bluish colour" , "Translucent gemstone" , "Shimmering stone" , "Gem-stone with a milky quality" , "Glassy gemstone" .)

I've seen this clue in The Times.

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