Carat weight measures a diamond's size. Each carat is equal to 100 points. Thus, a .75-carat diamond is the same as a 75-point stone or a 3/4-carat diamond. The larger a diamond, the more rare it is. Not only are larger diamonds found infrequently in nature, but they also show off the stone's color and cut to the best advantage.
The purer a diamond, the more brilliant it is. A diamond's clarity is judged by the number, color, type, size and position of inclusions - "nature's birthmarks" - it contains. The fewer inclusions the stone has - as determined by a grading system - the more valuable it is.
The purer the color of a diamond, the more rare it is. Truly colorless diamonds are the most valuable, while those with subtle yellow or brown tones - as determined by a grading system - are less so. Comparing stones side by side is the best way to see the color of each stone.
While nature is responsible for the other three Cs, it is the cut of a stone that releases its sparkle and beauty. When a diamond is cut to good proportions, light will reflect from one mirror-like facet to another and disperse through the top of the stone, resulting in a display of its brilliance.