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
jeweler may describe the weight of a diamond below one carat by its
'points' alone. For instance, the jeweler 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 else being equal, diamond price increases with
carat weight, because larger diamonds are more rare and more desirable.
But two diamonds of equal carat weight can have very different values
(and prices) depending on three other factors of the 4Cs:
It's important to remember that a diamond's value is determined using all of the 4Cs, not just carat weight.
. But two diamonds of equal weight can have very different values depending on their clarity, color, and cut. Carat weight is the most intuitive of the 4Cs – you expect a larger diamond to be worth more.
This right up is from the GIA web site