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Millimeters, by definition, have no carat weight.
Carat is the measurement of a diamond's weight. The stone's value would partially depend on its carat weight.
Carat is the weight measurement for gemstones.
A carat is a measurement of weight. The biggest carat, then, would be the weight of the biggest item being weighed using carats.
Carat is a weight, so its weight is one tenth of a carat or one tenth of .20 grams.