"Cut" refers to the proportion and the arrangement of a diamond's facets, and the specific quality of the workmanship. The reason "cut" is the most important of the four Cs is that it determines the amount of brilliance, sparkle, and fire in a diamond.
In the vast majority of diamonds, "color" actually refers to the absence of color. With diamonds, the less color in the stone the more valuable it is. While some of these subtle differences are not visible to the naked eye, they directly impact the quality and price of a diamond.
This refers to the purity of a diamond. Are there inclusions or blemishes in the diamond? How many and how large? The highest graded diamond will be considered "Flawless."
This is the category people seem most familiar with, the actual weight of a diamond. Generally, the greater the carat weight, the more expensive the diamond. Two diamonds of equal weight, however, can vary dramatically when one applies the other three Cs. That's why it's important to understand them all before you make your diamond purchase.
Photo not to scale.