Cut is the only quality of a diamond that is controlled by the human hand, in this case, the gem cutter, who shapes the rough crystal into a sparkling gem. The gem cutter's precision is critical because it is the cut of a diamond that dictates its brilliance and fire and releases its beauty. A diamond's quality of reflection - or brilliance - is tied to how well light is reflected and refracted within the stone.
Generally, light enters a diamond through the table, travels down to the pavilion where it reflects from one side to the other, and then reflects back up through the table to the eye of the viewer. The better proportioned the stone is and the more precise the angles of its facets are, the more light will be reflected. That is why, regardless of the shape of a diamond, each cut is created based on precise mathematical proportions.
The standard brilliant cut, still the most popular cut, was developed in 1919 and contains a total of 58 facets. Brilliance is an extremely important factor in judging the quality of a diamond. Even if a diamond has perfect clarity and color, if it is cut poorly - that is, too shallowly or too deeply - light will enter the table but instead of reflecting back, will get misdirected out the sides or bottom of the stone, resulting in less fire and brilliance.
On most laboratory reports there are grades assigned to polish and symmetry - components of the cut of the diamond and factors affecting brilliance. The major factors considered in the cut of a diamond are:
The size of the table in proportion to the diameter
The angle of the crown, which should be a minimum of 30 degrees, a maximum of 37 degrees from the horizontal.
The angle of the pavilion facet, which should be between 40 and 42 degrees from the horizontal. Note: The accepted angles deviate slightly from these numbers, depending on which cutting standard you are following.
The thickness of the girdle. This measurement varies from extremely thin to extremely thick with categories in between. The thickness of the girdle should not exceed more than 2% of the depth of the diamond.
To understand symmetry you have to consider the function of a diamond's cut. A diamond has to efficiently refract and reflect light back to the eye of the viewer. The light that enters a diamond, mostly through the table, is bent and mirrored throughout the stone and returned back to the eye. The facets of the diamond act as tiny mirrors, and when they are correctly positioned, return virtually all of the light that enters the gem. The better the symmetry, the more accurately the facets are positioned or related to each other, more of the entering light will come back out to sparkle and scintillate in the eye of the admirer. Symmetry denotes the correct relative relationship and lineup of the facets in the top section of the diamond (above the girdle) referred to as the crown facets, with the facets of the bottom section (below the girdle), called the pavilion facets. The accuracy of symmetry in the cutting of a diamond adds to the gem's sparkle and beauty.To get the most beautiful diamond one should buy a diamond with a symmetry rating of "good," "very good," or "excellent."
Among the most prominent diamond grading laboratories are: Gemological Institute of America (GIA) ; American Gem Society (AGS); Diamond High Council of Belgium (HRD) ; International Gemological Institute (IGI) ; European Gem Laboratory (EGL) These are the well-recognized laboratories. There are many other small accredited and non-accredited laboratories that are less well known.