Diamond Cut

Diamond Cut

Don't confuse the diamond's "cut" with the diamond's "shape."

First, don't confuse diamond "cut" with "shape." Shape refers to the general outward appearance of the diamond, (such as round, emerald, or pear). When a diamond jeweler (or a diamond certificate) says "cut," that's a reference to the diamond's reflective qualities, not the shape (or at least it should be, we have found that even some "jewelers" don't appear to know the difference between "cut" and "shape").

The quality of the "cut" does make a difference in how a diamond looks.

Diamond cut is perhaps the most important of the four Cs, so it is important to understand how this quality affects the properties and values of a diamond. A good cut gives a diamond its brilliance, which is that brightness that seems to come from the very heart of a diamond. The angles and finish of any diamond are what determine its ability to handle light, which leads to brilliance.
(See Diamond Anatomy for an explanation of the terms used in the next paragraphs.)
As shown in the images below, when a diamond is well-cut, light enters through the table and travels to the pavilion where it reflects from one side to the other before reflecting back out of the diamond through the table and to the observer's eye. This light is the brilliance we mentioned, and it's this flashing, fiery effect that makes diamonds so mesmerizing.

In a poorly cut diamond, the light that enters through the table reaches the facets and then 'leaks' out from the sides or bottom of the diamond rather than reflecting back to the eye. Less light reflected back to the eye means less brilliance. Good Proportions are Key
Most gemologists agree that the best cut diamonds are those that follow a set of formulae calculated to maximize brilliance. These formulae can be seen in a diamond's proportions, most importantly how the depth compares to the diameter, and how the diameter of the table compares to the diameter of the diamond.

If you opt to buy a diamond without an AGS certificate, spend some time looking at certified diamonds (where you know the Cut Grade) and train your eyes to identify the better cuts (by their "sparkle"). Cut does make a difference to the outward appearance of a diamond.

However, the variance in the proportions between an Ideal Cut and a Poor Cut can be difficult to discern by the casual observer.
Because cut is so important, several grading methods have been developed to help consumers determine the cut of a particular diamond. In general, these grades are:

• Ideal
• Premium
• Very Good
• Good
• Fair & Poor

Which Grade of Cut Should I Buy?
Selecting the grade of cut is really a matter of preference. To make the best selection, you need to understand the various grades. Please note that the descriptions below are general guidelines. Ideal Cut
This cut is intended to maximize brilliance, and the typically smaller table sizes of these diamonds have the added benefit of creating a great deal of dispersion or 'fire' as well. Ideal quality diamonds are truly for the person who enjoys knowing that he has one of the finest things that money can buy. This category applies only to round diamonds.
Premium
In the case of round diamonds, many Premium Cut diamonds have cuts that are the equal of any Ideal Cut diamond, though they often can be purchased at slightly lower prices than AGS Ideal Cuts. They are intended to provide maximum brilliance and fire. Like the Ideal Cut, these are also for the person who enjoys knowing that he has one of the finest things that money can buy.
Very Good
These diamonds reflect most of the light that enters them, creating a good deal of brilliance. With these diamonds, the cutters have chosen to stray slightly from the preferred diamond proportions in order to create a larger diamond. The result is that these diamonds fall slightly outside of some customers' preferences in terms of, for example, table size or girdle width, though, in many cases many of the parameters of diamonds in this range will overlap with certain parameters of diamonds in the Ideal or Premium ranges. Generally, the price of these diamonds in slightly below that of Premium cuts.
Good
Diamonds that reflect much of the light that enters them. Their proportions fall outside of the preferred range because the cutter has chosen to create the largest possible diamond from the original rough crystal, rather than cutting extra weight off to create a smaller Premium quality diamond. Diamonds in this range offer an excellent cost-savings to customers who want to stay in a budget without sacrificing quality or beauty.
Fair & Poor
A diamond graded as fair or poor reflects only a small proportion of the light that enters it. Typically these diamonds have been cut to maximize the carat weight over most other considerations.
Diamond Anatomy
Wondering what on earth is the diamond's pavillion? Table? Culet? The graphic and supporting text below explain the various "parts" of a diamond.

• Diameter
  The width of the diamond as measured through the girdle.
• Table
  This is the large, flat top facet of a diamond.
• Crown
  The upper portion of a cut gemstone, above the girdle.
• Girdle
  The narrow rim of a diamond that separates the crown from the pavilion. It is the largest diameter to any part of the stone.
• Pavilion
  The lower portion of the diamond, below the girdle. It is sometimes referred to as the base.
• Culet
  The tiny facet on the pointed bottom of the pavilion, which is the portion of a cut gem below the girdle.
• Depth
  The height of a gemstone, from the culet to the table. 

from:diamondbuyingguide