Throughout the world, diamonds are weighed in metric carats. There are five carats to one gram. Historically, this was the regular and uniform weight of the seed of the carob tree. The same measure was also used in the process of weighing gold to make up alloys, but there should be no confusion between carat as used in gold standards and carat as used in weighing stones.
The size of a stone is also an indication of its rarity. Larger stones, being much rarer than smaller ones, are far more valuable. Unlike other commodities that are priced by weight, a 2-carat diamond, for example, would cost more than two 1-carat stones of the same quality. Furthermore, a 2-carat stone is less than twice the diameter of a 1-carat stone.
Before the diamonds are set, they will have been weighed on an electronic balance that shows the weight to two decimal places. There are 100 points to a carat, so a 1/4-carat stone weighs 0.25 carats, or 25 points.
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These images are intended to show comparative sizes and are not lifesize