Light Performance Diamond Cut Grade

The AGS Light Performance Cut Grading system changed the industry with a new way of measuring cut, raising the bar for the highest-quality cut diamonds. Each skillfully cut facet serves a purpose and contributes to the essential characteristics that are the key elements to light performance and the final grade.

half aset and half diamond image

To understand the AGS Performance Cut Grading system, we must begin by considering why diamonds are cut at all. In their natural rough form, diamonds aren’t much to look at and are, therefore, rarely used in art and jewelry. Compared to a finely cut diamond, the rough crystal is dull and lifeless. Until the diamond is cut, all of its inherent beauty exists only as potential.

It takes the skill and expertise of a master diamond cutter to unlock the natural beauty of the diamond. This is why the cut of the diamond is so significant and is considered the most important of the 4Cs. It is the cutter who breathes life into the diamond and transforms it from a dull, lifeless rock into a work of art. It is the cutter who shows us the “diamond in the rough.”

An expertly cut diamond is much more than the marriage of art and nature; it is the culmination of centuries of human experimentation, imagination, and innovation. Nothing is easy with diamonds, and cutters always have faced many challenges. While most people are aware of the extreme hardness and rarity of diamonds, what is less known are the extreme design challenges with which cutters must deal. Diamonds are cut according to a design, and master diamond cutting is all about executing the design as precisely as possible.

Diamonds are finicky, and the penalties for mistakes can be severe. Even slight errors can mean the difference between a bright, lively diamond and a dull, mediocre commercial grade stone. Unfortunately, this presents a problem to the uninformed and inexperienced consumer who may not be able to tell the difference. The AGS Performance Cut Grade system was developed to solve this problem and, thereby, protect the consumer.

Think of a diamond as a tiny geometrical sculpture where every facet has a purpose, and the beauty (performance) of the sculpture depends on the precise alignment/angle of each individual facet.

Imagine what is happening when you are enjoying the dance of light across the diamond. Light from the surrounding panorama enters the diamond and moves through it according to the geometry of the sculpture. A complex kaleidoscope of light emerges—mathematics in motion.

A key insight from this thought experiment is the observation that EVERY facet has a job to do, so every facet contributes to the appearance of the diamond. The whole is the sum of the parts…and then some.

What this observation meant for the AGS is that old traditional methods for grading cut had to be abandoned in favor of a more scientifically rigorous approach. Traditionally, look-up charts were the primary means of cut grading. In this approach, all the complexity of the 3D sculpture was reduced to a handful of numbers representing the averages of various angles and measurements. These few numbers were then “looked-up” in grading charts to determine the cut grade.

three diamond images showing facet arrangement


3d model of a diamondThe problem with the traditional approach is that all the geometric flaws and asymmetries of the 3D sculpture are washed away during the averaging process, so even the most twisted and distorted geometry is indistinguishable from a perfectly cut stone. This is why the AGS looked to other solutions when developing the Performance Grading system. Specifically, if the appearance of the diamond depends on the full richness and intricacies of the 3D sculpture, then shouldn’t the Cut Grade be based on the same 3D sculpture, flaws and all?

light rays refracting in a diamond modelThe key underlying idea of the AGS Performance Grading system is the observation that EVERY facet contributes to the appearance of the stone. Using state-of-the-art technology, the system uses the actual 3D model of the diamond and performs a direct assessment of the diamond’s light performance from the model. Because every facet is considered in the assessment, every cutting mistake and design flaw can impact the final Cut Grade. Consequently, the AGS Performance Cut Grade system is the most technologically advanced and scientifically rigorous cut grade system in the world and represents a new paradigm in cut grading.

Of all the factors that contribute to the appearance of a diamond, what are the most important? What are the most universally agreed upon attributes of finely cut diamonds? What are the key elements of light performance?

The answers can be reduced to four properties that are regarded almost universally  as the essential characteristics of fine makes:

  1. Brightness. Finely cut diamonds tend to appear BRIGHT in a broad variety of lighting environments.
  2. Fire. Fine makes tend to exhibit large amounts of FIRE when illuminated in fire-friendly lighting environments.
  3. Leakage. Fine makes do not have excessively large areas of light LEAKAGE.
  4. Contrast. Fine makes will have enough dark/light CONTRAST to be interesting, but not too much dark as to impact the brightness of the stone.


diamond as viewed top downThink of it this way. Diamonds that exhibit these characteristics are doing successfully what diamonds are designed to do, in much the same way as an automobile that takes you from point A to point B is doing what it was designed to do. So, just as performance metrics can be created to measure how well an automobile performs (i.e. power, handling, acceleration), metrics  also can be created to measure how well a diamond performs. This is the common sense idea of what the light performance of a diamond is all about.  

 Note that scintillation is intentionally left off the list, as there is no universally accepted standard for how much scintillation a fine make should have. Some people claim that “more is more” and others claim “less is more.” Generally, there are trade-offs in diamond design between the size of the individual “sparkles” and the frequency/quantity of the sparkles. This is closely related to the “patterning” of the stone, which like scintillation falls within the art of the diamond cut and not the performance. In other words, these are taste factors and, therefore, do not belong in a performance cut grading system.   

Besides the four factors listed above, there are five other performance factors that encompass the AGS Light Performance Grade. These are more practical concerns related to the diamond’s performance and also have analogues in other types of performance grading systems in other industries.  

  1. Weight Ratio. Finely cut diamonds achieve a high level of performance without using excessive weight. 
  2. Durability. Fine makes are resistant to chipping or damage.
  3. Tilt. Fine makes do not exhibit the “fish-eye” effect.
  4. Girdle. Fine makes do not have excessively thin or thick girdles, mostly due to durability/weight ratio concerns.  
  5. Culet. Fine makes do not have an excessively large culet.


While all nine of these factors determine the “Light Performance” component of the AGS Performance Cut Grade, there exist two other important factors that make up the overall cut grade of the stone:

  1. Polish
  2. Symmetry

In this section, we’ve uploaded the Quality Assurance Letter template for AGS retailers to send to their suppliers. This letter is a tool you can use to assure your suppliers are in compliance with FTC guidelines.

Sometimes things can sound initially more complicated than they are in reality.  This is the case with the AGS Performance Cut Grade system. Yes, there are several patented technologies encompassing the system with enough technobabble to satisfy the nerdiest among us, and yes, the fine details of the system are mathematical and technical. However, the IDEAS behind the system are based on good old common sense. If you want to break out your pocket protector and get into the technical details of the AGS Performance Cut Grade system, then by all means, read the peer-reviewed articles, patents, and other publications. But if you’re a big-picture kind of person, you’re at the right place.   

What does a diamond do? What is its nature? What are you seeing when you look at a diamond?   

The answer is: beauty. What you see in a diamond is everything in the panorama surrounding the stone…including you! The bright sparkles you see are the lights above your head or the windows beside you, or even the light from your TV. The darker areas in the diamond may be the shirt you are wearing, the bookcase against the wall, or the smile on your face.

animation of the light path through a diamonddiamond facet arrangement

The main point is that the complex pattern of light and dark in the diamond is really a multifaceted view of the surrounding world. Look closely, and you will notice that the diamond is partitioned into myriad tiny regions, each a virtual window into the world and light around you. As the diamond or the viewer moves, the scenery changes. Bright lights are seen through some of the virtual windows, creating dazzling sparkles as the stone comes alive.

So what makes a diamond appear bright? 

There’s a simple (common sense) answer. Diamonds appear bright if the virtual windows are pointing toward the brightest areas of the surrounding world. This idea is the foundation of much of the AGS Performance Cut Grade system and is the essence of the “Light Performance Maps” (ASET® maps) included on the Platinum DQD® documents.

ASET Maps showing how brightness, contrast, and leakage are assessed

Where are the brightest areas of the world?

Look around, and you will observe that the world is lit from above. Therefore, diamonds with virtual windows pointing toward the higher places will tend to be brighter than those pointing to the lower. This is true, except for one caveat : If a virtual window is pointing too high, then it is your own face you will likely see and not the brighter world behind it. Interestingly, it is this obscuration effect of your own body and head that can create some of the most striking and visually appealing contrast patterns in a diamond. One such example is the familiar “arrows” pattern in the standard Round Brilliant. Not only can a diamond’s contrast pattern be visually interesting, it also  can contribute to the brilliance of the stone by making the adjacent regions in the diamond appear brighter.

There you have it. While the implementation of the AGS Performance Cut Grade system may be mathematical and technical, the underlying ideas are based on easy-to-understand concepts and straightforward observations. Simply put, diamonds appear bright if they are cut to draw light from the brightest areas of the world. Among other things, this idea implies that the brightest diamonds will exhibit a minimum amount of light leakage and will not draw too much light from high angles where the observer’s head can obscure light sources. While not a complete technical description of the AGS Performance Cut Grade system, these concepts, as well as the others discussed in this section, serve as the big-picture ideas behind much of the AGS Performance Cut Grade system.

The following links are cutting guideline charts.