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 gem fusion)

'old friends' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friends' becomes 'pal' (I can't justify this - if you can 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 Sydney Morning Herald and The Telegraph.
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