Diamonds are renowned for their ability to transmit light and sparkle so intensely. We often think of a diamond's cut as shape (round, emerald, pear) but a diamond's cut grade is really about the symmetry, proportioning and polish of a diamond and how well a diamond's facets interact with light. This means if it is cut poorly, it will be less luminous.
The cut of a diamond greatly affects a diamond's brilliance and is crucial to the stone's final beauty and value.
Of all the diamond 4Cs, it is the most complex and technically difficult to analyze.
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).
In simple terms - If a diamond is cut either too shallow or too deep then the light cannot refract correctly and does not sparkle as much.
The GIA grades a diamonds cut using the scale; Excellent, very good, good, fair, poor.
Hearts and arrows
During the 1980s in Japan, it was discovered that when a round brilliant diamond with exceptional symmetry was viewed from the bottom through a special viewer a pattern of hearts could be seen, and when viewed from the top showed eight gray arrowheads. Diamonds displaying such exceptional symmetry account for less than 1 % of all diamonds cut. The highest grades of polish, symmetry and perfect cutting angles allow it to reflect more light than a standard cut diamond.