The weight of a gem? (5)
I believe the answer is:
stone
I believe this is a double definition.
'the weight' is the first definition.
(stone is a unit of weight)
'a gem?' is the second definition.
(I've seen this in another clue)
'of' acts as a link.
(Other definitions for stone that I've seen before include "Rock; cherry seed" , "Seed of e.g. cherry" , "Rock; fruit seed" , "Pebble" , "See 21" .)