One of the "four c's” of diamond grading (5)

I believe the answer is:

carat

(Other definitions for carat that I've seen before include "Unit of weight for gems or purity of gold" , "Gem weight unit" , "Measure of weight of diamonds" , "Unit of weight of gemstones" , "A unit of weight for precious stones" .)

I've seen this clue in The Wall Street Journal.

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