Unit of weight for gems (5)
I believe the answer is:
carat
'unit of weight for gems' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for carat that I've seen before include "Jewellery unit" , "Measure applied to gold or gems" , "Gem weight unit" , "Unit of weight for diamonds" , "Diamond weight unit" .)