Lustrous gem (5)
(Other definitions for pearl that I've seen before include "greatly valued" , "Gem from oyster" , "prized example" , "Lustrous globule prized as a gem" , "Off-white colour" .)
'lustrous gem' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opal that I've seen before include "Type of gemstone" , "Gem-stone with a milky quality" , "Semiprecious stone of white and bluish colour" , "Iridescent quartz-like stone" , "Milk-coloured gemstone" .)