Old friend finding precious stone (4)
I believe the answer is:
opal
'precious stone' is the definition.
(I've seen this in another clue)
'old friend' is the wordplay.
'old' becomes 'o' (abbreviation).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'finding' acts as a link.
(Other definitions for opal that I've seen before include "Delicately-coloured gem" , "Translucent gem" , "Many-coloured gem" , "Form of hydrated silica, often used as a gemstone" , "Opaque gem" .)