Chum after ring and gemstone (4)
I believe the answer is:
opal
'gemstone' is the definition.
(opal is a kind of gemstone)
'chum after ring' is the wordplay.
'chum' becomes 'pal' ('pal' can be a synonym of 'chum').
'after' means one lot of letters go next to another.
'ring' becomes 'o'.
'pal' put after 'o' is 'OPAL'.
'and' is the link.
(Other definitions for opal that I've seen before include "Translucent gemstone" , "Milky-white gemstone with flashes of colour" , "Something milky-white" , "Type of gemstone" , "Semi-precious stone of b;uish white colour" .)