Cut
Cut is the most overlooked of the Four Cs of Diamond quality, and is very important as it effects a Diamond's beauty and value. A finer cut gives a Diamond more brilliance, which to the consumer is what a Diamond is really all about.
 

The particular angles and finish of any Diamond are what determine its ability to reflect light, which in turn, leads to brilliance. Various factors regarding cut are considered.

The consumer frequently confuses a Diamond's shape with its cut, most likely due to the industry's synonymous use of the term cut to describe a Diamonds shape (marquise cut, pear shape). When we refer to the cut of a Diamond, we are really referring to a compilation of two factors - its proportions in terms of angles degrees and percentages, and the qualities of its finish.
 

Viewing from left to right: A shallow cut, A good cut, An ideal cut
  
Proportion:
A well cut Diamond reflects light back to the eye evenly and very brightly, with no dark areas. As the diagram above illustrates, when a Diamond is properly proportioned and well-cut, light enters through the table and travels all the way to the pavilion (bottom half) where it reflects from one side to the other - intensifying in the mirror-like facets as it travels - before reflecting back out of the Diamond through the crown (top half) and to the observer's eye.
Finish:
Finish refers to the surface qualities given to a Diamond by the Diamond cutter. The term finish covers every aspect of a Diamond's appearance that is not a result of the intrinsic nature of the Diamond (when it comes out of the ground). When viewing a grading report, you will note that its finish is actually broken-down into two categories: Polish and Symmetry.

Polish:
Polish refers to the overall surface "finish" of a Diamond. The quality of the polish of a Diamond affects the ability of light to reflect from its facets.

Cutters sometimes encounter variations in hardness, or grain, as they polish a Diamond. This often results in microscopic polish lines running across a facet. These grain lines result in a lower rating for polish on a certificate, but rarely is the polish of Diamond so bad that it mars the beauty of a Diamond. Examples of blemishes that might be considered 'polish' characteristics are faint polishing lines and small surface nicks or scratches.

Symmetry:
Symmetry refers to the consistency of facet shape and how well the points of each facet align with each other. The actual shape and proportions of a Diamond are not considered when a Diamond is graded for symmetry.

A Diamond can have extremely poor proportions and still be rated as Very Good in symmetry if its facets are equal and the overall shape of the Diamond is balanced. Some of the more common symmetry faults that Diamond graders look for are, round-brilliant Diamonds that are "out-of-round", facets that are out of alignment, facet points that do not meet and an off-center table.

 

For Colored Gemstones the CUT grade is the third most important factor for evaluating colored gemstones. However, cut can positively or adversely affect the color grade of a gemstone as well. In addition, designer cut colored gemstones will add a premium to the overall value. Proportions taken in account are as follows: Outline balance, length to width ratio, profile balance, total depth percentage, crown height and pavilion depth, bulge, table size and brilliance. Keep in mind that most colored gemstones, rough yield is the primary factor in cut. Colored gemstone cutters try to get the largest finished gemstone possible from every piece of rough material.
Extra Fine:
Excellent proportions and finish, length to width ratio in preferred range, table size and girdle thickness in preferred range, polish and symmetry very good to excellent.
Fine:
Very good proportions and finish, length to width ratio should still be in preferred range or very close to it, table size and girdle thickness close to preferred range, polish and symmetry good to very good.
Good:
Good proportions and finish, length to width ratio may be out of preferred range, table and girde may be out of preferred range, polish and symmetry fair to good.
Fair:
Poor to fair proportions and finish, length to width ratio undesirable, obviously large table and girdle, polish and symmetry poor to fair.
 
Like the rest of the Four Cs, the influence of cut on value is directly related to its rarity.

As you read about other THREE of the Four Cs (Carat, Color & Clarity), you will see how cut plays an important role in Diamond and Colored Gemstone certification.