Iridescent (7)
I believe the answer is:
opaline
'iridescent' is the definition.
(I know that iridescent can be written as opaline)
This is the entire clue.
(Other definitions for opaline that I've seen before include "Opaquish glass; iridescent" , "Photographic plate" , "whitish glass" , "Translucent glass" , "like a stone" .)