Part 3: The Idea of Performance and the Nine Factors Comprising the Light Performance Grade

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.

Think 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.  

5. Weight Ratio. Finely cut diamonds achieve a high level of performance without using excessive weight.

6. Durability. Fine makes are resistant to chipping.

7. Tilt. Fine makes do not exhibit the “fish-eye” effect.

8. Girdle. Fine makes do not have excessively thin or thick girdles, mostly due to durability/weight ratio concerns. 

9. 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