Cut

Cut refers to the pattern of angled facets on the surface of the stone. Do not confuse cut with shape, shape refers to the physical form or contour of the stone, while cut is fine combination of features like symmetry, depth, crown and pavilion angles, uniformity of facets, quality of polish and the overall skill of the cutter who is sculpting the stone.
Diamond Cut

Facets act as mirrors reflecting light that enters the stone. This essentially means more the reflection, better the brilliance. In a well cut diamond, facets are arranged such that of all the light that enters the stone, maximum amount is reflected, enhancing brilliance.
Diamond Facets

Width and depth play a huge role in the way light travels in the gem. A well cut diamond can create a beautiful brilliance, and a poorly cut diamond may appear dull. In a shallow cut or a deep cut diamond the brilliance is dulled because of the lack of proper reflection of light within the facets.
About Diamond Cut

Styles of Cut
Irrespective of the shape, a stone can be cut in different styles depending on a number of factors including physical structure of the rough crystal, the inherent color, clarity and of course popular demand. Traditionally, diamonds were enjoyed in their inherent shape with only polishing of the crystal faces as the basic cut. The detailing and designing of the stone's cut evolved with time. In modern times, the most popular styles of cuts can be characterized as:

Brilliant Cut - Brilliant Cut maximizes the brilliance and fire of a diamond. A brilliant cut diamond has more facets (57 or 58 if the culet facet is also included) as compared to other cut styles. Brilliant cut is the most popular style of diamond cut. Usually brilliant cut applies to round stones; other shapes in this cut style include marquise, heart, trillion, oval and pear.

Step Cut - Step Cut consists of linear facets arranged parallel to the girdle, resembling a set of steps. This cut style accentuates the color, clarity and luster of a diamond with a subtle brilliance as the number of facets is less. The cut applies to stones that are square, rectangle or trapezoid shaped.

Mixed Cut - Mixed Cut couples the best of both brilliant and step cuts; it combines the dimensions of step cut with the brilliance of brilliant cut. In a Mixed Cut, typically the crown is brilliant cut and the pavilion is step cut. The most popular shapes within the mixed cut category are princess, radiant and cushion.

Grading Cut
As compared to the other characteristics of a diamond, Cut is the most difficult to grade. This difficulty is because Cut is a combination of a number of aspects including crown depth, crown angle, pavilion depth, pavilion angle, symmetrical arrangement of facets, polish amongst others; a perfect balance is difficult to determine. Although the variance in the proportions between an Ideal Cut and a Poor Cut can be difficult to discern, the industry cut grading standard helps in determining the quality of the Cut.
Grading Cut

A cut can be classified as:

Excellent Cut/Ideal Cut - Perfectly proportioned with the highest grade of polish and symmetry, these stones reflect virtually all the light that enters. They are the most brilliant, rare and one of the finest diamonds money can buy.

Very Good Cut - Usually cut to maximize the size of the diamond, a Very Good Cut stone falls a little short of the preferred diamond proportions. A Very Good Cut Diamond reflects most of the light that enters the stone, creating a good deal of brilliance.

Good Cut - When a cutter chooses to create the maximum possible size diamond out of the rough crystal, the proportions of the stone fall out of the ideal range. Much of the light that enters the stone is reflected back, exhibiting a good brilliance. An ideal choice for shoppers who want to stay in budget without sacrificing quality or beauty

Fair and Poor Cut - A diamond in this grade reflects only a small proportion of the light that enters it. These gems are created to maximize carat weight over other considerations.

Symmetry & Polish
Symmetry refers to the appropriateness of the shape and facet arrangement of the stone - how identical and exact the size as well as placement of each facet is. Symmetrical imperfections include off-center table, wavy girdle, misshapen or extra facets, asymmetrical girdle outline, crown and pavilion mismatch, off-center culet amongst others.
Symmetry & Polish

Polish refers to the quality of finish or smoothness of the facets. Polish defects make the surface of the diamond appear dull and blurred eventually dimming scintillation.
Diamond Polish