Weight of a gem? (5)
I believe the answer is:
stone
This clue is a double definition.
'weight' is the first definition.
(stone is a unit of weight)
'a gem?' is the second definition.
(I know that gem can be written as stone)
'of' is the link.
(Other definitions for stone that I've seen before include "Little rock" , "that one will 26d at 8a [FLING at HIGHLAND GAMES]" , "Small rock" , "Gem; Staffs. town" , "Beryl, ruby" .)