Gemstone weight unit (5)
I believe the answer is:
carat
'gemstone weight unit' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for carat that I've seen before include "Unit of weight for diamonds" , "Unit of gem measurement" , "Measure of fineness of gold" , "Measure of purity of gold" , "A unit of weight for precious stones" .)