Cut is determined by how a diamond's facets interact with light. More than any other factor, cut determines the beauty of the stone.
No matter the size of the diamond, the quality of the cut will maximize the stone's brightness and sparkle. A poorly cut diamond will appear dull even if it has a high color and clarity grade.
Diamond clarity is a measure of the purity and rarity of the stone. A flawless stone will have no visible inclusions (internal flaws) and no blemishes (external imperfections) when viewed under 10-power magnification.
A diamond with a poor clarity grade has multiple inclusions, which directly affects its sparkle and makes it appear cloudy.
Color is the second most important of the 4Cs because it directly affects the stone's appearance. In diamonds, color actually refers to an absence of color. The best “color” is no color at all.
Each diamond is evaluated by a gemologist against a pre-graded set and assigned a letter grade from "D" (colorless) to "Z" (light yellow). Diamonds with a poor color grade can appear more yellow instead of the desired brilliant white.
Carat is the weight of a diamond, not the size. Taken by itself, carat weight does not determine a diamond's value.
Carat weight can appear differently across different diamond shapes. For example, a diamond may have a higher carat weight without seeming larger. Two diamonds of the same carat weight can vary in size if one is cut more deeply than the other.