Down Under gem (4)
I believe the answer is:
opal
'down under 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 "Opaque gem" , "often semi-transparent" , "Gem with delicate colours" , "Milky iridescent stone" , "Opaque mineral used a gemstone" .)