Natural gemstones not right for rings (5)
I believe the answer is:
peals
'for rings' is the definition.
('peal' can be a synonym of 'ring')
'natural gemstones not right' is the wordplay.
'natural gemstones' becomes 'pearls' (pearls are gemstones).
'not' suggests deleting specific letters.
'right' becomes 'r' (abbreviation).
'pearls' with 'r' taken out is 'PEALS'.
(Other definitions for peals that I've seen before include "Leaps about with bursts of laughter" , "Lapse in bell sounds" , "Loud sounds of bells or laughter" .)