Old friend is a gem (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'old friend' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'is a' is the link.
(Other definitions for opal that I've seen before include "what may be set in 29 [DIAL]" , "One's stony" , "Shimmering stone" , "Semi-precious stone of milky or bluish colour" , "Milky stuff that's hard" .)