Iridescent stone (4)
I believe the answer is:
opal
'iridescent 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 "Milky stuff that's hard" , "Type of gem" , "Type of gemstone" , "precious jewel" , "Semiprecious stone of white and bluish colour" .)