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.

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