A diamond's cut is considered to be the most important of the four Cs. It is important to understand how a diamond's proportions and the relationship between them affects its brilliance, fire, and scintillation.

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The cut of a diamond not only refers to the diamond’s shape, it also refers to how effectively the diamond returns light back to the viewer’s eye. A well-cut diamond will appear very brilliant and fiery, while a poorly cut diamond can appear dark and lifeless, regardless of its colour or clarity. Not only do well-cut diamonds appear more brilliant, they also tend to appear larger than other diamonds of the same carat weight.

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A well-cut diamonds exhibit three different properties: brilliance, dispersion and scintillation. As light strikes a diamond's surface, it will either reflect off the table of a polished stone or enter the diamond. The light that is reflected off the diamond is known as the diamond's brilliance. As light travels through a stone, some of the light rays are separated into flashes of colour. This is known as dispersion. The result of dispersion—the separation of white light into its spectral colours—is known as fire. Scintillation is flashes of colour that are viewable as an observer moves a diamond back and forth.

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Some Diamonds exhibit a phenomena called Hearts and Arrows. Seen from above, a Hearts and Arrows diamond shows an arrow pattern. When displayed on its pavilion side, the diamond presents the eye with 8 hearts with tiny 'v' shapes. Genuine Hearts and Arrows have these patterns visible at a single glance, indicating that the diamond has perfect optical symmetry.