Diamond carat weight is the measurement of how much a diamond weighs.
A metric "carat" is defined as 200 milligrams.
Each carat can be subdivided into 100 'points.' This allows very precise measurements to the hundredth decimal place. A jeweller may describe the weight of a diamond below one carat by its 'points' alone. For instance, the jeweller may refer to a diamond that weighs 0.25 carats as a 'twenty-five pointer.' Diamond weights greater than one carat are expressed in carats and decimals. A 1.08 carat stone would be described as 'one point oh eight carats.'
All things being equal, diamond price increases with diamond carat weight, because larger diamonds are rarer and more desirable. But two diamonds of equal carat weight can have very different values (and prices) depending on the other diamond 4Cs: Clarity, Colour, and Cut.
How did the carat system start?
The modern carat system started with the carob seed. Early gem traders used the small, uniform seeds as counterweights in their balance scales. The carat is the same gram weight in every corner of the world.
The word "carat" was possibly derived from the Greek word kerátion literally meaning a small horn, and refers to the carob seed as a unit of weight.
What are "magic sizes"?
Some weights are considered "magic sizes" – half carat, three-quarter carat, and carat.
Visually, there is little difference between a 0.99 carat diamond and one that weighs a full carat.But the price differences between the two can be significant.