Old friend's a gem (4)
I believe the answer is:
'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?" .)