The unit of weight since 1913 is the metric carat, with fractions expressed decimally in points (rarely now in grains), except that less valuable stones are sold by the metric gram; synthetic gemstones by the size in millimetres; pearls by carats and grains (1 carat = 4 grains); and cultured pearls formerly by the momme. As a diamond that is brilliant cut is of a prescribed shape, its weight can be closely estimated (as is usually done if it is in a setting) by its diameter at the girdle, e.g. a stone with a diameter of 6mm (about ¼-inch) weighs approximately 1 carat, and there are standard schedules for other sizes.
The unit of weight now is the metric gram or kilogram (1000 grams), replacing the use of the Troy ounce (oz) which equalled 20 pennyweights or 480 grains.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson