Generally speaking, there is some agreement on how round brilliant-cut diamonds should be cut to optimize brilliance and dispersion. However, there is no universal standard as to what constitutes the "ultimate" or "perfect" proportions for a round brilliant. This is a current area of research for EGL USA and the diamond industry.

The cut of a diamond--its roundness, its depth and width, and the uniformity of the facets--all determine a diamond's ability to exhibit brilliance.

The width and depth have the greatest effect on how light travels within the diamond, and how it exits in the form of brilliance. As cutting quality can be a confusing subject, ask your professional jeweler about "ideal" proportions and request a cut grading report from a major independent gemological laboratory like EGL USA.

Diameter: The width of the diamond as measured through the girdle.

Table: The flat top and largest facet of a gemstone.

Crown: The top portion of a diamond extending from the girdle to the table.

Girdle: The narrow band around the widest part of a diamond.

Pavilion: The bottom portion of a diamond, extending from the girdle to the point of the stone.

Culet: The facet at the tip of a gemstone. The preferred culet is not visible with the unaided eye (graded "medium" or "none")

Depth: The height of a gemstone measured from the culet to the table.


How does cut affect fancy shaped diamonds?
What about fancy cut diamonds like marquise, pear, oval, heart, emerald, princess, radiant, and others? How can a consumer know a fancy-cut diamond is well cut?

Due to the symmetry of round brilliant-cut diamonds, it is much easier to formulate proportion criteria and strike an optimum balance between brilliance and dispersion. Although EGL USA supplies extensive proportion information on its diamond certificates for fancy-cut diamonds, there is currently no universal set of standards constituting ideal proportions for these shapes. With that said, here are some general guidelines:

• The degree of brilliance in a fancy shape can be one way to tell whether it is cut within acceptable proportions or not. A fancy-cut diamond should be exciting to look at and should dance in the light --
  it shouldn't suffer from too much "light leakage" through the pavilion.
  
  
• A phenomenon called the "bowtie" effect - an obvious diminished area of brilliance appearing like a bowtie - occurs in certain fancy-shape diamonds when the proportions are off. Well-cut fancy shape diamonds show only a minimal bowtie effect. This is something your eye will be able to decide.
  
  
• The symmetry of fancy cut diamonds is an important consideration. Take a look at how well the two halves of a fancy shape diamond look when seen from the profile view and the top view. The two halves should display very little differences, and ideally be mirror images of one another. Diamonds with mismatched halves may have been cut that way to save weight.
  
  
• Diamonds with points at their ends, such as marquises, pears, and hearts, should be thick enough at the points to stand up to normal wear and tear. If these diamonds are cut too shallow, a point may be vulnerable to chipping under certain circumstances.