The brightness and sparkle of a diamond are what will draw you to the jewellery store and are what will keep you captivated long after you’ve made your purchase. Maximum  brightness and sparkle are the result of excellent cutting technique, which is reflected in the diamond’s cut grade. Of the 4 C’s Cut is by far the most important as it has the most impact on a diamond’s appearance.

The ‘cut’ of the diamond is often misinterpreted as its shape rather than its cut quality. Cut is all about the diamond’s fire, sparkle, brightness and as a factor that adds value, refers to a diamond’s proportions, symmetry and polish.

Although no single proportion measurement is an indicator of what the diamond’s cut grade will range from Excellent, Very Good, Good, Fair or Poor, a number of complex factors are considered when deciding on a cut grade. While every individual facet matters in deciding the cut grade, the appearance of the diamond is a result of a combination of all its proportions.

Table size: The horizontal facet at the top of a diamond is called the table. The average table size is expressed as a percentage of the diamond’s average girdle diameter. An ‘Excellent’ grade diamond will have a table size between 52 and 62 percent.

Total Depth refers to the diamond’s depth from the surface of the table to the culet.

The Pavilion is the lower portion from the bottom edge of the girdle to the culet. Expressed as a percentage of the average girdle diameter, a Pavilion Depth that’s too deep or too shallow will allow light to escape from the side or bottom of the stone. A well cut diamond tends to direct more light upwards through the crown.

As this dimension relates to the diamond’s brightness, Pavilion Angle is a very important aspect. It is the average of the angles formed by the stone’s pavilion main facets and girdle plane. In order to be graded ‘Excellent’ the Pavilion Angle should be between 40.6 & 41.8 degrees along with other parameters being in their proper ranges.

Crown Height: From the girdle’s top edge to the table lies the crown of the diamond. The average crown height is generally expressed as a percentage of the average girdle diameter. It may affect both, the dispersion and brightness of a diamond.

Crown Angle: The angle that is formed where the bezel facets meet the girdle plane. The Crown Angle has a significant effect on the face up appearance of a round-brilliant diamond such as the one in the picture. Apart from offering contrasting routes for entering light, the best crown angles also provide a route for exiting light dispersion.

Girdle Thickness: The middle portion of the diamond which separates the crown from the pavilion is known as the girdle. The girdle serves as the diamond’s setting edge. The girdle thickness refers to the range from its thinnest to its thickest area. A medium to slightly thick girdle is usually recommended as a thick girdle adds unnecessary weight to the stone making the diamond appear smaller. In contrast, a very thin girdle, occasionally referred to as a knife edge results in a diamond that is extremely fragile and prone to chipping.

Lower Girdle/ Half Facet Length: This ratio is calculated by measuring how long the lower girdle facets are, relative to the length of its pavilion. Diamonds with longer half facets will tend to have higher scintillation.

Culet: The small facet located at the bottom of the diamond to prevent it from chipping and abrasion is called the Culet. Sizes vary from none, very small, small, medium, slightly large, large, very large and extremely large. When the culet is absent, it is referred to as a pointed culet.

Source: Gemological Institute of America