Milky white stone
I believe the answer is:
'milky white 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 "Type of gem" , "Milky-white gem" , "Milky white gem" , "Translucent gem" , "Semi-precious stone of b;uish white colour" .)