The cut is mainly influenced by the harmony between the table and depth percentages and crown and pavilion angels, either causing the diamond to dissipate light (poor cut) or optimally refract and reflect light (excellent cut). The table and depth percentages are calculated as follows:
Depth Percentage: The higher the number, the deeper the stone. The lower the number the shallower the stone.
Table Percentage: The higher the number, the bigger the table looks. The lower the number, the smaller the table looks.
Table and depth percentages affect how light travels within the diamond and impacts a stone’s brilliance. If a cut is too shallow, light escapes out from the sides and the diamond loses brilliance. If the cut is too deep, light is lost from the bottom and the diamond appears dull or dark.
GIA have developed a grading system, which take these and other important factors into account to provide a scientific assessment of a diamond’s sparkle. Seventy Seven Diamonds uses the GIA classification of Excellent, Very Good, Good, Fair and Poor (not displayed on the website) grades to help customers assess diamond quality. The table below displays the GIA grading classifications relating to cut.
|Excellent (or Ideal)
Diamonds classified as Excellent or Ideal cut offer the most sparkle, dispersion and fire and are considered to be of the highest quality. Light moving through an ideal cut diamond bounces almost fully back out the top of the stone and brings its brilliance into view.
Diamonds classified as Very Good offer slightly less brilliance and scintillation than the excellent cut. However, as the difference between the two is almost indistinguishable to the unaided eye, very good cuts offer better value for money than excellent cut diamonds.
Diamonds classified as “Good” usually allow some light to escape during the reflective process, although once again, the difference between this and the very good cut diamonds is small therefore good cuts offer great value for money and make for a more affordable option.
Light moving through a shallow cut diamond is lost out of the bottom of the stone and the lack of light play makes shallow cut diamonds appear lifeless.
Light moving through a deep cut diamond escapes out from the sides, darkening all or most portions of the stone.
The poor sparkle performance of Diamonds classified as Fair or Poor cut may be noticeable to the untrained eye, and are for this reason also less costly.
Other factors affecting cut
When the facets of a diamond are well balanced and aligned, the stone has symmetry, which is critical to creating optimal brilliance and scintillation. Errors in symmetry include facets that are not properly aligned, improperly shaped facets, or an off-centre table.
After a diamond is cut, each facet must be polished. The process can leave surface scratches or marks, which are like streaks left behind after a car is waxed. If a diamond has no scratches or very minor ones, the polish is of a high degree. Scratches, lines, burn marks created by excessive heat, or rough girdles could downgrade the polish rating of a diamond and if significant, could affect the overall cut grading.
The relationship between the crown and the pavilion angles has an effect on the appearance of a diamond. A diamond's pavilion angle and depth must be correct to capture and reflect light optimally. A slightly steep pavilion angle can be complemented by a shallower crown angle, and vice versa.
In diamonds with extremely deep pavilions, the whole surface of the table appears to be darker creating what is known as a “nailhead”. Gems with more shallow pavilions often produce a “fisheye” effect due to the girdle’s reflection in the diamond’s table.