Pawn nobleman's gemstone (5)
I believe the answer is:
pearl
'gemstone' is the definition.
(I know that pearl is a type of organic gem)
'pawn nobleman's' is the wordplay.
'pawn' becomes 'p'.
'nobleman' becomes 'earl' (member of the nobility).
'p'+'earl'='PEARL'
(Other definitions for pearl that I've seen before include "Gem; an attacked Harbor" , "Object of great worth" , "finest example" , "Gem from a bivalve" , "Smooth lustrous round structure of calcium carbonate" .)