Ring from New York with unknown gem (4)
I believe the answer is:
onyx
'gem' is the definition.
(onyx is a kind of gem)
'ring from new york with unknown' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'from' is a charade indicator (letters next to each other).
'new york' becomes 'ny'.
'with' is a charade indicator (letters next to each other).
'unknown' becomes 'x' (algebra).
'o'+'ny'+'x'='ONYX'
(Other definitions for onyx that I've seen before include "Banded gemstone" , "Stone-like marble with layered colours" , "Type of agate" , "Stone-like marble with different colours in layers" , "Agate with different bands of colour" .)