Diamond cut has the greatest impact on diamond brilliance. If a diamond is well cut, it will appear more luminous because light reflects with more radiance, fire, and sparkle. Cut, the arrangement of facets, is often confused with diamond shape such as round, heart, oval, marquise, or pear.

As a standard, three "ideal cuts" are commonly used to compare the shapes and appearances of diamond crystals based on their faceted arrangement. Currently, the round brilliant cut is the most popular diamond shape and cut, which relies on mathematical and empirical analysis to perfect its facet arrangements.

To rate the quality of a diamond's cut, a trained grader evaluates the cut compared to standards. Those stones that closely match the symmetry and proportions of the benchmark are given higher grades. A diamond grader evaluates the craftsman's skill in determining the quality of the cut. Because diamonds are valued in part by their weight, a diamond cutter wants to cut the heaviest diamond from a rough stone. A desire for the heaviest weight and ideal cut can sometimes conflict, resulting in a lower cut grade.

Advanced technologies such as laser cutting and computer-aided design have enabled cuts with unprecedented complexity, optical performance, and waste reduction, but also allowed for unusual shapes such as stars and butterflies.

The preciseness of the diamond cut will allow a diamond to reflect light perfectly. This is so that the stone's radiance, fire, and sparkle are at their absolute finest.

The best natural diamond cut is graded Excellent by GIA, and the best lab diamond cut is graded Ideal by IGI. Excellent (GIA) and Ideal (IGI) cut diamonds possess the most precise angles, symmetry, proportion, balance of facets, and the best polish that a highly skilled diamond cutter can achieve. 

The diamond cut grade also considers the diamond's weight in relation to its diameter, the thickness of its girdle (which affects its durability), and the quality of polish on its facet arrangement.

Diamond cuts are classified using the GIA Diamond Cut Scale for round brilliants in the D-to-Z diamond color range, which has five grades Excellent (EX), Very Good (VG), Good (G), Fair (F), and Poor (P). IGI uses a Diamond Cut Scale with five grades Ideal, Excellent, Very good, Good, and Fair.

All clarities across the D-to-Z color range are graded according to a standardized Cut Grading System.

Knowing how light behaves inside a diamond helps us understand diamond cuts. There are three main ways to classify diamond sparkle.

Dispersion, or fire, separates colored light into its full spectrum. While white light enters and exits through the table, flashes of color are visible when the light is emitted from the center. This is done through the crown.

Brilliance. The amount of white light emitted and reflected out of a diamond determines its brilliance. Diamonds absorb natural light, then split it into the full spectrum of colors thanks to their wide top table. As the light bounces back and reflects within the diamond, it is projected upwards and leaves the table as pure white light.

Scintillation. A diamond's scintillation refers to how light bounces off its facets when moved, creating a glistening and twinkling effect. Light will emanate from diamonds when they are well cut, creating the mesmerizing sparkle we all love.

An Excellent diamond cut will enhance the appearance of other qualities of the stone, like the color and clarity grade. It can even make a diamond appear larger. When a diamond is cut to an Excellent (GIA) or Ideal (IGI) grade, the polish and proportions of the stone will produce brilliance, fire and scintillation that work together in harmony, creating a charmingly sparkly diamond.

The only diamond shape to receive a ‘cut grade’ are round brilliant cut diamonds. However, you should ensure any other diamond shape has Excellent symmetry and polish grade, or at least Very Good. ‘Brilliant cut’ diamonds are designed to exude the most sparkle. Step cuts feature larger, parallel facets that celebrate the clarity of the stone with more subtle flashes of light

The round brilliant diamond is the most sparkly when graded triple Excellent (GIA) or Ideal (IGI). The diamond will have the optimal cut, symmetry, and polish. Multiple lines of symmetry and 58 facets work together to reflect light within and out of the diamond, making the round brilliant the sparkliest diamond shape. Typically, round brilliant diamonds are the most expensive diamond cuts, due to three factors; their unmatched brilliance, popularity, and the amount of diamond waste generated during cutting Round brilliants are the most popular shapes.

Due to their higher sparkle, brilliant cut diamonds can appear larger than step cut diamonds. The spread of the table makes some brilliant cut shapes appear larger than others; marquise, oval, and pear diamonds appear larger than other diamond shapes of the same carat weight.

It is more difficult to find some diamond shapes than others. The demand for asscher diamonds and radiant diamonds is much lower than for other diamond shapes, making them rarer than most diamond cuts. They are followed closely by marquise and hearts.

Due to many of the reasons already stated, the round brilliant diamond continues to be the most popular diamond shape in the United States. Followed by oval diamonds, pear diamonds, emerald cut diamonds, and cushion cut diamonds.