Old friend gets stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'old friend' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'gets' acts as a link.
(Other definitions for opal that I've seen before include "One in a ring perhaps" , "Gem stone" , "it may be highly valued" , "Semi-precious stone of milky or bluish colour" , "National gemstone of Australia" .)