Lustrous stone (4)
I believe the answer is:
opal
'lustrous stone' 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 "Semi-transparent gemstone" , "Opaque gem" , "Something milky-white" , "Semi-precious stone of b;uish white colour" , "Gem-stone with a milky quality" .)