Quiet and noble - a gem (5)
I believe the answer is:
pearl
'a gem' is the definition.
(I know that pearl is a type of gem)
'quiet and noble' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'and' is a charade indicator (letters next to each other).
'noble' becomes 'earl' (earl is a kind of noble).
'p'+'earl'='PEARL'
(Other definitions for pearl that I've seen before include "something highly valued" , "finest example" , "Much loved and valued person" , "piece of wisdom" , "Lustrous structure inside an oyster-shell" .)