Crossword Genius

Work on a large gemstone (4)

Ross

I believe the answer is:

opal

'gemstone' is the definition.
(opal is a kind of gemstone)

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

(Other definitions for opal that I've seen before include "Milky-white gemstone with flashes of colour" , "Variegated gemstone" , "Milky-white gem" , "One's stony" , "Mineral, usually iridescent" .)

I've seen this clue in The Times.
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