Cut

Cut is often thought to be the most important of the four Cs, because a good cut reflects light, giving the diamond its fire and sparkle. The cut is not the shape of the diamond, but refers to the cutting of the diamond into facets and the way in which it reflects light internally. The shape of the diamond, such as round, emerald, or pear, refers to the overall appearance of the diamond. The cut is the primary factor in the brilliance or fire of a diamond where light seems to come from the very heart of the diamond itself.

A well-cut diamond has light entering through the table and traveling to the pavilion where it reflects from one side to the other before reflecting back out of the diamond, through the table, to the observer's eye. This is the brilliance or fire of a diamond you can see easily, making the diamond appear to flash and mesmerizing our fancy!

In these diagrams you can see that a deep-cut diamond may have a higher carat weight than an ideal cut (it weighs more) but it will not have the same brilliance and fire and is a much less desirable diamond. Poorly cut diamonds do not reflect the light back to the viewer properly, leaking light from the edges and bottom of the diamond.
The brilliant cut diamond is cut with 58 facets, 33 on the crown and 25 on the pavilion. These facets should be uniform and symmetrical or the diamond's ability to reflect light is affected. When buying a diamond, ask about the cut. Many jewelers will not discuss cut unless asked; a poorly cut diamond costs a lot less than a well-cut diamond, often making it look like a great bargain. However, the diamond is inferior and does not possess the sparkle we expect from a diamond.
Diamond Proportions

It is often difficult for the average person to tell the difference between an ideal cut and a poor cut with a quick examination. To help grade diamonds according to cut, gemologists use a set of formulas that dictate the angles and facet sizes in an ideal diamond. The amount by which the diamond’s cut deviates from this ideal provides the lower grades.
Cut may be one of the most important factors in a diamond’s appeal, so gemologists have defined several grading scales. The most widely used grades are:

Ideal: Offers the maximize brilliance possible. The smaller table sizes of ideal cut diamonds create a great deal of dispersion (“fire”) within the stone. Ideal cut diamonds are for the discerning purchaser who knows that they have one of the finest diamonds money can buy. The Ideal cut category applies only to round diamonds.
Premium: In round diamonds many Premium cuts are the equal of any Ideal cut, though they can be purchased at slightly lower prices. Premium cut diamonds provide maximum brilliance and fire. Like the Ideal cut, these diamonds are also for the person who enjoys the finest diamonds money can buy.
Very Good: These diamonds reflect most of the light that enters them, offering a great deal of brilliance. Often with Very Good cuts, the diamond cutters chose to stray slightly from the preferred diamond proportions in order to create a larger stone. These diamonds fall outside of the perfect table size or girdle width, although in many cases the cut range overlaps with the parameters of diamonds in the better ranges. The price of these diamonds is slightly below those of Premium cuts.
Good: Good cut diamonds reflect much of the light that enters them but their proportions fall outside the preferred range so the cutter can create the largest possible stone from the original crystal. Diamonds in this range provide the buyer with excellent cost-savings without sacrificing quality or beauty.
Fair & Poor: These diamonds reflect only a small part of the light that enters them. These diamonds are usually cut to maximize carat weight over other considerations. These cuts are often found in retail mall jewelry stores.

Brilliant or Round Diamonds

A brilliant cut diamond usually is cut with 58 facets (33 on the crown and 25 on the pavilion). These facets will be uniform and symmetrical on a well-proportioned stone or the diamond’s ability to reflect light will be compromised.
When trying to choose a cut for a diamond, you need to look at the depth percentage and the table percentage (see diagram below). The depth percentage is the depth of the diamond (table to culet) divided by the width of the diamond. The higher the depth percentage, the deeper the stone appears. For a round diamond the best depth percentage for an ideal cut is 58-62.9%.
The table percentage is the length of the table divided by the width of the diamond. The higher this number, the bigger the table looks. For a round diamond, the best table percentage for an ideal cut is 53-57%.

It is important not to confuse a small table size with a small diameter. In general, you should look for a bigger diameter and a smaller table percentage for the most beautiful round diamonds.