Unit of gold purity (5)

I believe the answer is:

carat

(Other definitions for carat that I've seen before include "Gem weight unit" , "Gemstone weight unit" , "Unit of measurement for the proportion of gold in an alloy" , "Unit of weight for gems or purity of gold" , "Weight of ice" .)

I've seen this clue in the Evening Standard, The Mirror and The Times.

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