Gems of quality brought aboard ship (6)
I believe the answer is:
stones
'gems' is the definition.
(I've seen this before)
'quality brought aboard ship' is the wordplay.
'quality' becomes 'tone' (synonyms).
'brought aboard' means one lot of letters goes inside another (inserted letters come inside or on board).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'tone' inserted inside 'ss' is 'STONES'.
'of' acts as a link.
(Other definitions for stones that I've seen before include "''Sticks and ...... may break my bones''" , "Onsets with imperial weights" , "Attacks" , "Small pieces of rock" , "Weights and pieces of rock." .)