Carat

The weight of Diamonds and Gemstones is measured in CARATS, often abbreviated "ct.". One carat is 200 milligrams, 5 carats equals 1.00 gram. One carat is divided into 100 parts, called "points". Therefore, a 25-point Diamond weighs ¼ or 0.25 of a carat, and a 50-point Diamond weighs ½ or 0.50 ct. etc.
Diamond and Gemstone weights are usually expressed in carats and decimals. A 1.09 ct. stone, for example, would be described as one point o nine carats. A Diamond that weighs 0.99 ct. is said to weigh ninety-nine points.

The price per carat of Diamonds on a World Gemmological Laboratory , as with any Diamond, will increase with size; all other factors being equal. Therefore, a 0.50 ct. Diamond will cost more than twice that of an equal quality 0.25 ct.

While most people would like to own a large Diamond, it is important to keep in mind that to determine a Diamonds value, a variety of factors must be taken into consideration. Cut, clarity, color and carat weight, not one of them being any more important than the others, all have to be considered together in determining the true value of a Diamond.
            The proportionate increase in size of round-brilliant Diamonds (Not actual size)
 
Large Diamonds and Gemstones are more rare than smaller Diamonds or Gemstones. The more scare the Diamond or Gemstone is, the greater its worth. So a larger stone costs more per carat. As you read about the other THREE of the Four Cs (Clarity, Color & Cut), you will see how carat plays an important role in Diamond and Gemstone certification.
 
World Gemmological Laboratory will make its best estimate of the value and size of all gemstones and diamonds in a given jewelry item, but cannot guarantee exact size due to the constraints of grading and measuring stones that are already in jewelry.