Ring friends for gemstones (5)
I believe the answer is:
opals
'gemstones' is the definition.
(opal is a kind of gemstone)
'ring friends' is the wordplay.
'ring' becomes 'O' (O is the shape of a ring).
'friends' becomes 'pals' (pal is a kind of friend).
'o'+'pals'='OPALS'
'for' is the link.
(Other definitions for opals that I've seen before include "Translucent gemstones" , "Milky-white stones" , "Milky gemstones with delicate colours" , "Semi-precious stones of milky or bluish colour" , "Iridescent gems" .)