The 4C's: Diamond Cut Grading & Shape Explained

round brilliant diamond shining on white background

Diamond Cut Grading and Shape Explained

How Does a Diamond Interact with Light?

A diamond interacts with light in 3 ways:

Reflection:

some of the light  hits the diamond crown and facets and  is reflected back out and this is seen as white flashes of light or BRILLIANCE. A diamond with optimal proportions and cut will reflect more light that it allows to enter and “leak” out of the sides of the precious stone

Refraction:

 light entering the stone changes its path, or is refracted. The optimal situation here is refraction back out through the table of the Diamond

Dispersion:

light that has entered the Diamond can also be dispersed into its constituent colours, a rainbow effect. These intense flashes of colour including red are know as FIRE

Diamond Cut

The cut of a Diamond refers to its proportion, symmetry and polish. Diamond Cut has a huge impact on the optical properties of a Diamond, hence quantifying and grading cut quality is critical when appraising precious stones.

Currently the GIA only assess Round Brilliant Diamonds for their cut, and currently no universally accepted method exists to appraise Fancy Cuts in a similar manner. A cut grade from Excellent-Poor is assigned, and this correlates with the optical properties of the Diamond. But what parameters are used to measure this?

diamond anatomy diagram

The diagram above (courtesy of the GIA) shows the anatomy and various parts of a Diamond.

Marcel Tolkowsky analysed and quantified the ideal proportions of a Round Brilliant Diamond and this work in turn has set the standard for GIA and other laboratory grading.

The table below explains some of the main metrics used to measure and grade Diamond Cut:

 Measurement  Definition Excellent-Ideal Proportions 
Table Size Table Area/ Average Girdle Diameter * 100 52-62 %
Total Depth Crown to Culet/ Average Girdle Diameter *100 61%-62.5%
Pavilion Depth Distance from Girdle to Culet/ Average Girdle Diameter *100 42.8%-43.2%
Pavilion Angle Average angles formed by the diamond’s pavilion main facets and its girdle plane 40.6-41 degrees
Crown Height Distance from top of girdle to the table as percentage of average girdle diameter 40.6%-41.8%
Crown Angle The crown angle is the angle that’s formed where the bezel facets meet the girdle plane 31.5-36.5 degrees
Girdle Thickness Narrow band that is border between crown and pavilion Thin
Lower Girdle/ Half Facet Length A measurement of how long the lower girdle facets are relative to the length of the pavilion. 65%-90%, longer lower half facets will have more scintillation
Culet Small facet at the bottom of a diamond intended to prevent chipping and abrasion to the point None- V small, should not be visible

 
Ideal Cut Proportions

The above values are indicative, with some grading laboratories being more stringent than others, and "Ideal Cuts" also being assessed against higher standards. The individual values can be used to filter out lesser diamonds, but shouldn't be used in isolation, as all the surfaces and internal aspects of a Diamond interact with one another.

When looking at a Diamond Certificate we would advise checking the various proportions, as even all excellent cuts aren't created equal. You will note that all percentage values are calculated as a function of the Diamonds average girdle diameter.

Examples of how Proportions and Diamond Cut impact interactions with light include:

  • Table Percentage; When the table percentage is low, light is trapped inside the diamond and leaks out the sides of the diamond (instead of reflecting back through the table). Conversely when table percentage is too high, light doesn’t reflect off of the diamond’s crown angles and facets—leaving the diamond looking dull. GIA Excellent values range from 52-62 percent
  • Depth Percentage; A diamond with a lower depth percentage usually appears larger due to its increased width, but often creates a dark appearance as the diamond doesn’t reflect light as well. A diamond with too high of a depth percentage loses light out the bottom of the diamond, making it appear dull.
  • The pavilion is the lower portion of a diamond from the bottom edge of the girdle to the culet. The pavilion depth is expressed as a percentage of the average girdle diameter. A pavilion depth that’s too shallow or too deep will allow light to escape from the side or bottom of the stone. A well-cut diamond will direct more light upward through the crown.
  • The crown angle ideally should range from 31.5 to 36.5 degrees. The crown angle has a large effect on the face-up appearance of a Round Brilliant Cut Diamond. The best range of crown angles provide a route for exiting light dispersion, as well as additional contrasting directions for entering light.
  • Pavilion angle is another important dimension of the stone, especially as it relates to a diamond’s brightness. It is the average of the angles formed by the diamond’s pavilion main facets and its girdle plane. This should fall between 40.6 and 41.8 degrees to be considered “Excellent,” providing other parameters also fit their proper ranges.

Diamond Girdle Thickness

A factor less critical re optical properties but also important is the girdle thickness. An overly thick girdle will add unnecessary weight to a Diamond, increasing its price with little aesthetic benefits, whereas an extremely thin girdle can be prone to chipping and very fragile. 

    The closer to ideal values the cut is, the better the optical properties and appearance of the Diamond.

    Diamond Shape

    A diamond’s shape refers to its physical appearance and not to be confused with diamond cut, which has been discussed above.

    The majority of diamond shapes are Round, Square (such as the Cushion), or Rectangular (for example Asscher, Princess, Emerald, Radiant).

    Many other shapes exist, with all shapes other than the Round Brilliant being known as "Fancies" or Fancy Cuts, and these include Pear-Shaped, Oval, Marquise or Heart-Shaped Diamonds.



    The most popular diamond shape is the Round Brilliant, followed by the Princess Cut (second most popular). 

     What to consider when choosing your diamond shape

    Elongated shaped diamonds such as Oval, Pear, or Marquise are best suited to women with shorter fingers. When choosing a band try to avoid wide bands that create the illusion of even shorter fingers.

     


    Square shaped Diamonds such as Princess and Radiant are best suited for women with long fingers, helping to create the appearance of slightly shorter fingers.

    Round and Heart Shaped Diamonds are more flattering for larger or wider fingers. In these instances its also prudent to avoid narrow diamond shapes such as Marquise and Emerald as well as delicate ring settings

    This article is a brief overview, and we have dedicated pages with detailed information on many diamond shapes and styles. Should you have any further questions feel free to Contact Us.