Gold purity unit (5)

I believe the answer is:

carat

(Other definitions for carat that I've seen before include "Measure of weight of diamonds" , "Unit of weight used for gems" , "Measure of fineness of gold" , "Unit of measurement for the proportion of gold in an alloy" , "Unit of weight of gemstones" .)

I've seen this clue in the Evening Standard and the LA Times.

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