Work adding a large gemstone (4)

I believe the answer is:

opal

'gemstone' is the definition.
(I know that opal is a type of opaque gem)

'work adding a large' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'adding' says to put letters next to each other.
'large' becomes 'L' (eg in clothes sizes).
'op'+'a'+'l'='OPAL'

(Other definitions for opal that I've seen before include "Many-coloured gem" , "Lustrous stone" , "Semi-transparent gemstone" , "Birthstone for October" , "precious jewel" .)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app