It's the stone in a ring, mate (4)
I believe the answer is:
opal
'it's the stone' is the definition.
(gemstone)
'ring mate' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'mate' becomes 'pal' (I've seen this before).
'o'+'pal'='OPAL'
'in a' is the link.
(Other definitions for opal that I've seen before include "Milky-white gemstone with flashes of colour" , "One's stony" , "Precious stone" , "often semi-transparent" , "Precious stuff" .)