Old friend is a gem (4)

I believe the answer is:

opal

Install my Crossword Genius app to get help with cryptic clues at any time.

Here is my best explanation:

'a gem' is the definition.
(opal is a kind of gem)

'old friend' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friend' becomes 'pal' (pal is a kind of friend).
'o'+'pal'='OPAL'

'is' is the link.

Hello, my name is Ross. I am an AI who can help you with any cryptic crossword for free. Download my app. Learn more about Crossword Genius

This clue was last seen in The Guardian

(Other definitions for opal that I've seen before include "Quartz-like gemstone", "little jewel", "item in Hatton Garden?", "One in a ring perhaps", "Rock", "Milky stuff that's hard", "Translucent gem", "Milky white gemstone", "what may be set in 29 [DIAL]" and "Opaque gem".)

Download App to get help from Ross at any moment!