Its a stone for the ring, mate (4)

I believe the answer is:

opal

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'its a stone for' is the definition.
I can't judge whether this defines the answer.

'ring mate' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'mate' becomes 'pal' (I've seen this in another clue).
'o'+'pal'='OPAL'

'the' is the link.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Variegated gemstone" , "Delicately coloured gem" , "Milky-white gem" , "Glassy gemstone" , "Milky iridescent stone" .)

I've seen this clue in The Irish Independent.

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