Crossword Genius

Old friend’s a gem (4)


I believe the answer is:


I'm a little stuck... Click here to teach me more about this clue!

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

'old friends' is the wordplay.
'old' becomes 'o' (abbreviation).
'friends' becomes 'pal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Delicately-coloured gem" , "Milky-white gem" , "Rock" , "Quartz-like gemstone" , "item in Hatton Garden?" .)

I've seen this clue in The Telegraph, The Times and the Sydney Morning Herald.
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