Carat (mass) for gemstone
The carat is a unit used for measuring gems and pearls, and is exactly 200 milligrams. The expression came to English from French, derived from the Greek kerátion, "fruit of the carob". Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight.

Because most gemstones are expensive, it is necessary to use subdivisions of the carat. It is usually subdivided and recorded to 2 decimal places, a hundredth of a carat is known in rather unmathematical trade jargon as a "point". Half a carat is therefore written .50 carats or 0.50 carats, and would be referred to as 50 points.

Carat (purity) for gold & platinum
The carat (abbreviation ct or K) is a measure of the purity of gold and platinum alloys. In the United States and Canada, the spelling karat is now solely used for the measure of purity, while carat solely refers to the measure of mass weight.

24 karat (millesimal fineness 999)

22 karat (millesimal fineness 916)

20 karat (millesimal fineness 833)

18 karat (millesimal fineness 750)

15 karat (millesimal fineness 625)

14 karat (millesimal fineness 585)

10 karat (millesimal fineness 417)

9 karat (millesimal fineness 375)

Grain
A metric or pearl grain is equal to 50 milligrams or 1/4 of a carat as used in pearl and diamond nomenclature.

Momme
A unit of weight previously used in Japan to weigh pearls. 75 grains = momme

Troy
A system historically used for measuring the weight of precious metals and gemstones. The name comes from the French city of Troyes. 480 grains = 1 ounce troy

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