Gem weight (5)
I believe this is a double definition.
'gem' is the first definition.
(I've seen this before)
'weight?' is the second definition.
(stone is a unit of weight)
(Other definitions for stone that I've seen before include "Rag" , "Gem; Staffs. town" , "See 19" , "Hard lump of mineral matter" , "Beryl, ruby" .)
'gem weight' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for carat that I've seen before include "Jewellery unit" , "weight of rocks" , "unit employed among rocks" , "Measure of weight of diamonds" , "Gem weight unit" .)