Iridescent gemstone (4)

I believe the answer is:
opal
'iridescent gemstone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for opal that I've seen before include "Milky stuff that's hard" , "Lustrous stone" , "Precious stone" , "Type of gemstone" , "Iridescent gemstone" .)