CARAT
The last "C" to mention is carat.
A carat equals 2 gram and is a unit of measure or weight. There are 100 points to a carat, for example 25 points (0.25) is 1/4 carat or 50 points (0.50) is 1/2 carat and so on.
The weight is very important when pricing and selling a stone. A 0.98 carat stone is not in the same price category for the "price per carat" as a 1.0 carat diamond. While the size difference is not visible, the 1.0 carat gem has a higher price per carat than the 0.98 carat stone. A 2.0 carat diamond is much more expensive than 2 diamonds of 1.0 carat each. The larger the stone, the higher is the price per carat for the same quality. Most people aspire to own at least a 1.0 carat diamond.

All these 4 C's are important to understand a diamonds value.
A diamonds grading report is an expert's opinion of the quality of a diamond. This report will contain information about identification, enhancements, shape outline, measurements and the four C's, colour, cut, clarity and carat weight.
Having a report from an internationally accredited laboratory, such as the GIA or AGS, protects the consumer.

Prices for colourless diamonds are published in the "Rapaport Diamond Report" price-lists. There is however no official price-list for fancy-coloured diamonds.

The world record price for a white diamond or jewel of any kind was set by "The star of the Season", a magnificent pear-shaped diamond of 110.10 carats. This superb gem of the finest quality (D colour, internally flawless) was sold in public auction in May 1995 for $ 16.548.750 or $ 165.322 per carat.

The world record price for a fancy blue diamond was set by a marvellous rectangular-cut stone of 20.17 carats for $ 9.902.500 or $ 490.952 per carat. It was sold in public auction in October 1994.

And finally do not forget: