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Importance of Table, Bezel & Pavilion Mains

Some content on this page are contributed by Good Old Gold, with thanks.

A diamond's table, bezel facets and pavilion mains are the 3 most important types of facets that determine the diamond's light return.

When these 3 are cut to the correct proportions, most of the rays of light entering it will be refracted, internally reflected, and returned to the observer.

To achieve this, the pavilion angle is a key determining factor since the internal reflections occur at the inner pavilion surface. The table size and the crown angles are important as well because they determine the angle of light exit.

Pavilion Angle

The pavilion angle is the average angles that the 8 pavilion mains make with the horizontal plane. The job of the pavilion is most important of all: to reflect light. The light enters the diamond through the table and the facets of the crown, passes through the diamond, and is reflected back by the facets of the pavilion. Here's the important part: The angle of the pavilion for a round diamond must be between 40-41.5 degrees. If the pavilion mains are not cut to the correct angles, the diamond will not reflect the light properly, and the diamond will be less bright. In a round diamond, there is a dramatic loss of brightness if the pavilion angle is outside the acceptable range.

Here is an illustration of a ray of light entering and exiting at correct angles:

Shallow Pavilion

If the pavilion is cut too shallow, light will leak out from the diamond as follows:

Deep Pavilion

If the pavilion is cut too deep, light will leak out from the side of the diamond as follows:

Crown Angle

The crown angle is measured as the average angle that the bezel facets make with the horizontal plane. The angle of the crown is also important because it is the point of entry (and exit) of light from the face of the diamond. To qualify as ideal, the angle of the crown should be 33.7-35.8 degrees. If it is smaller than 32 degrees, the diamond is what we call spread-cut. This makes the table area too large and the girdle too thin, and a thin girdle affects the structural integrity of the diamond since it is prone to chipping. If the angle of the crown is above 35 degrees, it makes the diamond "top heavy." This extra weight at the top (which must be taken from elsewhere) results in a smaller diameter, making the diamond look smaller than its carat weight suggests.

It is important to know that the crown angle should have an inverse relationship with the pavilion angle. The ideal proportions as calculated by Marcel Tolkowsky is 34.5 degrees crown paired with 40.75 degrees pavilion. However, a diamond with 34 degrees crown and 41 degrees pavilion will be just as brilliant.

Therefore, it is not so critical to insist on a particular crown angle. But you must make sure that it compliments the pavilion angle.

Table Size

Table size is measured as the ratio of the table width as compared to the total width of the diamond. For rounds, the largest diameter is used. For example, if the table measures 5.7 mm, and the total diameter of the diamond measures 10.0 mm, then the table percentage is 57%.

It is generally agreed that the table of an ideal cut diamond should be between 53% to 58%. Within this range, a diamond is capable of the best brightness and fire.

It is interesting to note that many people actually prefer diamonds with a larger table. This is because having a large table means that the diamond has a shorter crown and is therefore lighter. Thus, with the same amount of carat weight, you can get a bigger diamond (at the expense of brightness and fire).

However, in recent years, ideal cut diamonds are much sought after. Hence diamond cutters try to keep within the ideal range (as specified by AGS and GIA) to maximise their profit. This is not a bad thing since as consumers, we also like our diamonds bright and fiery.

What table size and crown/pavilion angle combination?

As consumers, we may not know what crown angles are complimentary with what pavilion angles. Not to mention, the choice of table size, which is somewhat linked to the crown angle as well.

This is an age old problem that has no simple solution. The range of complimentary angles and table sizes are quite large, and not bounded by some range or mathematical formula. This is why AGS and GIA could not successfully grade the cut proportions until fairly recently.

Fortunately for consumers, diamond reports by AGS and GIA post January 2006 do a good job at weeding out diamonds with bad performance due to poor cut proportions. So, if we pick a diamond that that is within the AGS Ideal and GIA Excellent cut grading, we can be fairly certain that the pavilion angle, crown angle and table size offer good light return. However, we must be aware of the limitations of the AGS and GIA cut grading methodologies and not rely on them blindly. They are very good, but far from perfect.

The Holloway Cut Advisor (HCA)

The HCA and all information regarding it are intellectual properties of Garry Halloway.

The Holloway Cut Advisor is a petent pending tool developed by Garry Halloway. It evaluates the visual appearance of a round diamond as a combination of four factors - Light Return, Fire, Scintillation and Spread. Garry Halloway developed the HCA based on the visual appearance of the diamond from a 16 inch (40cm) distance, and only a 15° viewing angle (at the crown) is considered. He then built a database that can be used to check the quality of light return.

This is a good system for checking the table size and the combination of the crown/pavilion angles This is because the inputs of the HCA are: Total Depth %, Table % and Crown and Pavilion angles or %.

The grading scale is: 0-2 Excellent, 2-4 Very Good, 4-6 Good, 6-8 Fair, and 8-10 Poor. A score of zero is almost impossible because many of the factors conflict.

I must emphasise that a low HCA score does not gaurantee a performing diamond because it is based on how human observation correlates with these scores (similar to GIA's cut grading method) which has its inherent weaknesses and it does not consider the measurements of the minor facets. So, it will not check for girdle digging or painting, nor will it check the optical symmetry and patterning of the diamond. Therefore, although Garry Holloway uses the terms like Tolkowsky Ideal Cut (TIC) to describe the results of the HCA, these diamonds may be very far from ideal. More information is required to arrive at that conclusion.

Also, the HCA should not be used to rank diamonds. For example, a diamond with a score of 0.5 may not be superior to one with a score of 1.8. Within the ideal range, taste comes to play when we decide whether one diamond looks better than the other. What Garry and his test subjects think is a better looking diamond may not be the one you like. So, it is better to study the information presented by the vendor and view the diamond up close. Then choose the diamond that appeals to you.

Never-the-less, if you limit your diamond purchases to those with HCA scores that are under 2, you would have ensured that the table and crown/pavilion angles are sound, and that is it.

The Story So Far

We have described what is required of the 3 most important facet types of a diamond.

With the pavilion mains at the correct angles, light coming from the face of the diamond will be able to enter, and get reflected off the pavilion. With that settled, let us turn our attentiont to the crown of the diamond.

Light return from the table (coloured blue) is largely dependent on the pavilion angles since the table is essentially flat. If the crown angle is the correct, then light will be able to enter and exit from the bezel facets (coloured yellow) as well. The star facets (coloured green) are bound by the bezel facets and the table. So, they must be at the correct angles too. So, we have covered all the coloured areas (blue, green and yellow) on the crown of the diamond. If we can ensure good light return from all these facets of the crown, we have covered light return from most parts of the diamond's face.

So, let us move on to cover the light return from the upper and lower girdle facets in the next page.