Gemstone of silver polished off (5)
I believe the answer is:
agate
'gemstone' is the definition.
(agate is a kind of gemstone)
'silver polished off' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'polished off' becomes 'ate' (I've seen this in another clue).
'ag'+'ate'='AGATE'
'of' acts as a link.
(Other definitions for agate that I've seen before include "Semi-transparent stone" , "Quartz gem" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "Variegated chalcedony" , "Semi-precious quartz" .)