Birthstone after sapphire (4)
I believe the answer is:
'birthstone after sapphire' 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 "Rock" , "Semi-precious stone of b;uish white colour" , "Many-coloured gem" , "Quartz-like gemstone" , "what may be set in 29 [DIAL]" .)