Australia's national gemstone (4)
I believe the answer is:
opal
'australia's national gemstone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opal that I've seen before include "Many-coloured gem" , "Milky-white gem" , "Form of hydrated silica, often used as a gemstone" , "often semi-transparent" , "Gem-stone with a milky quality" .)