Weight of a gem? (5)
I believe the answer is:
stone
I believe this is a double definition.
'weight' is the first definition.
(stone is a unit of weight)
'gem?' is the second definition.
(I know that gem can be written as stone)
'of a' acts as a 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" .)