Australian gem (4)
I believe the answer is:
opal
'australian gem' 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 "Lustrous stone" , "Gem with delicate colours" , "Milky iridescent stone" , "Milky-white gemstone with flashes of colour" , "it may be highly valued" .)