Stone ring seen north of China (4)
I believe the answer is:
opal
'stone ring' is the definition.
'opal' can be an answer for 'stone' (gemstone). I'm not certain of the 'ring' bit.
'north of china' is the wordplay.
'north of' becomes 'o' (I am not sure about the 'north' bit.).
'china' becomes 'pal' (china plate is Cockney rhyming slang for mate).
'o'+'pal'='OPAL'
'seen' is the link.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Quartz-like gemstone" , "Many-coloured gem" , "Delicately-coloured gem" , "One in a ring perhaps" , "Milky-white gemstone with flashes of colour" .)