Milky stone (4)
I believe the answer is:
opal
'milky stone' 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 "Milky-white gem" , "Milk-coloured gemstone" , "One in a ring perhaps" , "often semi-transparent" , "Lustrous stone" .)