The GIA Diamond Cut Grading System for standard round brilliants in the D-to-Z colour range is based on the assessment of seven components. The first three — brightness (the total light reflected from a diamond), fire (the dispersion of light into the colours of the spectrum) and scintillation (the pattern of light and dark areas and the flashes of light, or sparkle, when a diamond is moved) — are appearance-based aspects. The remaining four — weight ratio, durability, polish and symmetry — are related to a diamond's design and craftsmanship.
In GIA’s system, each component is assessed individually, taking into account the relative importance of that component in the overall cut quality of the diamond. Each cut grade, based on a relative scale from Excellent to Poor, represents a range of proportion sets and face-up appearances. There are many different proportion sets that produce attractive diamonds.
For example, look at a side view of the standard round brilliant. The major components, from top to bottom, are the crown, girdle and pavilion. A round brilliant cut diamond has 57 or 58 facets, the 58th being a tiny flat facet at the bottom of the pavilion that’s known as the culet. The large, flat facet on the top is the table. The proportions of a diamond refer to the relationships between table size, crown angle and pavilion depth. It is important to note that a wide range of proportion combinations are possible, and these ultimately affect the stone’s interaction with light and how attractive the diamond is to the person viewing it.
While it is important to consider many components when assessing the overall cut appearance and quality of round brilliant diamonds, an individual's preferences also play a role. Because each cut grade represents a wide range of proportion sets, individuals have the freedom to choose which particular appearance they prefer within the grade range.
The diamond industry as well as the public can use cut along with colour, clarity and carat weight to help them make more informed decisions when assessing and purchasing round brilliant diamonds.
CUT VS. SHAPE
People often use the words cut and shape interchangeably. They think of cut as the shape or outline of the diamond, rather than the arrangement of facets needed to create an attractive face-up appearance.
Round is the shape used in most diamond jewellery. All other outlines are known as fancy shapes. Examples of traditional fancy shapes include the marquise, pear and oval. Hearts, triangles and a variety of other shapes are also gaining popularity in diamond jewellery.