Old friend is a gem (4)

I believe the answer is:
opal
Here is my best explanation:
'a gem' is the definition.
(opal is a kind of gem)
'old friend' is the subsidiary indication.
'old' becomes 'o'.
'friend' becomes 'pal' (pal is a kind of friend).
'o'+'pal'='opal'
'is' is the link.
(Other definitions for opal that I've seen before include "Semi-transparent gem", "Stone", "Milky white gem", "Type of gem", "Gemstone mineral", "Milky-white gem", "Delicately coloured gem", "Semi-precious stone of b;uish white colour", "Mineral used as a gemstone, of hydrated silica" and "Milky iridescent stone".)

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