Semiprecious gem (4)
I believe the answer is:
opal
'semiprecious gem' 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 "raw material" , "Milky white gemstone" , "Milky stuff that's hard" , "Rock" , "One's stony" .)