Gem's special quality (5)
I believe the answer is:
stone
'gem's' is the definition.
(thesaurus)
'special quality' is the wordplay.
'special' becomes 's' (common abbreviation as in Special Air Service).
'quality' becomes 'tone' ('tone' can be a synonym of 'quality').
's'+'tone'='STONE'
(Other definitions for stone that I've seen before include "Rag" , "Piece of rock - weight" , "Small rock" , "Little rock" , "Beryl, ruby" .)