Milky-white gemstone (4)
I believe the answer is:
opal
'milky-white gemstone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for opal that I've seen before include "Jewellery item" , "Precious stone" , "Iridescent quartz-like stone" , "Translucent gem" , "Quartz-like gemstone" .)