When a diamond interacts with light, every angle and every facet affects the amount of light returned to the eye. The sparkle of a diamond, then, has everything to do with how well the cut of the diamond was planned and executed. Although a diamond’s value is based on its unique combination of color, clarity, cut, and carat weight (the 4Cs), a diamond’s cut is the most complex to assess, and it’s the only component that can be directly affected by the diamond cutter.

Because there are so many variables and options to consider when cutting a diamond, the diamond cutter’s choices (whether man or machine) can produce two otherwise identical diamonds with vastly different levels of beauty, quality and value. This is particularly true of the standard round brilliant cut diamond.

After years of scientific research and real-life observation, GIA researchers proved that while every individual facet matters, a diamond’s appearance arises from the combined contribution of all its proportions. Therefore, for round brilliant diamonds, GIA considers how a diamond’s proportions relate to each other, rather than considering individual proportions in isolation.

GIA’s cut grade also considers the design and craftsmanship of the diamond, including its weight relative to its diameter, its girdle thickness (which affects its durability), the symmetry of its facet arrangement, and the quality of polish on those facets.

A magnificent diamond achieves its beauty through three optical effects:  Brightness (sometimes mistakenly called “Brilliance), Fire, and Scintillation.

  • Brightness refers to all the diamond’s internal and external reflections of white light.
  • Fire is the word used to describe the flashes of color you see in a polished diamond.
  • Scintillation describes the flashes of light and the contrasting dark areas you see when the diamond, the light, or the observer moves—a combination of sparkle (spots of light) and pattern (arrangement of lights and darks).

The diamond cutter’s decisions determine these three optical effects, and when taken together with the color, clarity, and carat weight produce an individual grade of quality for a buyer’s consideration.