The cut of a diamond is the physical measurement and relative proportion of a polished diamond and is the most important characteristic in producing a diamond's beauty. A single number does not define cut. Instead, it is a myriad of measurements, relative percentages, angles, finish, and performance of light within the diamond.

The brilliance and sparkle of a diamond is the result of the reflection and refraction of light within a diamond and is the cumulative effect of the many facets on the surface of the three-dimensional diamond shape. What makes judging cut difficult is that beauty is in the eye of the beholder. Different people perceive the beauty of a diamond differently in terms of what they perceive as beautiful.

In recent years, major diamond grading laboratories like the Gemological Institute of America (GIA) and the American Gem Society (AGS) have established cut grades to help the consumer make better decisions for purchasing diamonds based on cut characteristics. However, the cut grading systems are different from each other and constantly changing. As a result, there is no single official cut grade but the current systems are a giant leap ahead of the past with no cut grades.

The quality of the cut is most visible in terms of how light performs in a diamond to produce Brightness, Fire, and Scintillation.

Brightness is the visual effect of all the internal and external reflections of white light. This brightness (also known as brilliance) results primarily from the angles of the facets and the relative size of the top facet called the table.

Fire is the visual effect of rainbow colored flashes of light caused by the separation of white light into various colors when the diamond acts as a prism.

Scintillation is the visual effect of sparkle and patterns of light and darkness. Sparkle is the points of light that flash as the diamond, the light source or the observer moves. The arrangement and contrast between the bright and dark areas is important to what the eye perceives as beautiful. An all-bright diamond or an all-dark diamond is less attractive than a diamond with a balanced, symmetrical and contrasting pattern of reflection.

Some of the more important diamond measurements influencing the cut grade are depth percentage, table percentage, crown angle, and pavilion angle. The initial cut grade research focused on round brilliant diamond shapes but the grading laboratories are slowly introducing cut grades for the fancy shapes like the princess, emerald, oval, radiant, cushion and others. Each diamond shape has its own set of cut parameters that produce the optimal light performance and beauty for that particular diamond shape.

If the cut of the diamond is the key to its beauty, why would anyone purchase a diamond without great cut? Probably the main reason is lack of knowledge by the consumer. Until recent years, it has been difficult for the consumer to assess the cut of the diamond because jewelers focused on color, clarity and carat weight. The jeweler used the bright lights in the showroom to mask the light performance of the diamond. As the diamond shopper becomes more aware of what constitutes cut and has access to the GIA or AGS cut grades on the diamond grading reports, they will be able to filter out the average and poor cut diamonds in their purchase decisions.

Sometimes budget is a factor in purchasing something less than the best cut. However, cut is typically a real bargain since there is little visual difference between the top echelons of diamond cut and the price difference between average and exceptional cut is often minimal. Perhaps the real cost of getting better cut in a diamond, is the time and effort to learn about cut and to find a retailer who can provide diamonds with exceptional cut at reasonable prices.

Once a diamond shopper has determined what shape of diamond they want to purchase, they need to research what cut parameters contribute to desirable appearance. Some diamond shoppers fall into a trap of narrowly defining their acceptable range for every diamond measurement, not realizing that the beauty of the diamond results from the interrelationships of all the facets. Simply picking the middle of the range for each parameter can lead to a mediocre result. The cut grades are an attempt to rate how a combination of factors work together in the diamond.

Regardless of the cut grade systems today or in the future, only the diamond shopper can determine what is most appealing to their eye. Cut is critical to the beauty of the diamond so making the best diamond purchase decision means understanding the importance of cut and finding that special diamond that is beautiful to behold.