The qualities that make a diamond so visually appealing and beautiful are sparkle and brilliance. While the 4c's. are very important, the quality of a diamond's Cut in particular is very important to creating brilliance, sparkle, and flashes of fire.
Brilliance consists of two components, brightness and contrast. A well-cut diamond. will have brightness. As you move closer to the diamond you will see its contrast pattern become more apparent (areas of light and dark which compliment each other). A diamond that is cut too deep or too shallow has a tendency to allow light to leak out the back of the stone. Diffused lighting does not produce fire or scintillation so it's good for judging color and clarity, and visible inclusions that may be masked by the lighting. If you're analyzing color, be sure to keep a white wall in front of you, since a diamond picks up and reflects colors around it. Diamonds that are bright return light from their surroundings back to the viewer.
Dispersed light that appears as flashes of rainbow colors is known as the fire of a diamond. You can see more fire in dark places, such as restaurants or clubs where there are few lights. A well-cut diamond, regardless of shape, should have a healthy balance of colored sparkle, especially in direct spotlighting. Several factors affect the amount of fire in a diamond; star facet length, lower girdle facet length, pavilion angle, facet junctions, the angle at which light enters the diamond, and the angle of the light rays as they exit the diamond.
When a diamond sparkles intensely as it moves, this is known as scintillation. Scintillation is well seen in a flood-lit or office-lit environment, where the natural fire of a diamond seems nonexistent. High-end jewelry stores often have bright lights reflecting from walls, small directional spots and additional pinpoint LED or fiber-optic lighting positioned inside the case. In such an environment, you can see sparkle just by changing your position relative to the diamond. If you rock it back and forth in your hand, a well-cut stone. will put on a fireworks display of brightly colored and white sparkles, enhanced by shifting contrast. Preferably, a diamond has many satisfying flashes that are spread across the surface of the stone, and usually has very few dull places.
Polish is the factor that affects how well light is able to pass through a diamond and is crucial to the brilliance of a diamond. Polish is graded by laboratories using the terms ideal, excellent, very good, good, fair and poor. It is important to choose a diamond that is graded with a good, very good, or excellent polish. Poorly polished facets may reduce the intensity of light reflected from, or refracted into and out of a diamond. Labs assess polish by examining the diamond, facet by facet, with a microscope. The average person may not see any difference - but there is a price difference.
Symmetry can be an important aspect to your diamond. Symmetry means that there is an exact reflection of form on the diamond's shape and arrangement. Symmetry affects the diamond cut and can have an affect on its value and brilliance as well. Symmetry is the overall look of the diamond.
According to the AGS, with an ideally cut diamond, the stone must be placed at precise angles and contain precise proportions. This placement normally ensures an ideal balance between the maximum amount of brilliance and dispersion of light. Any divergence from these proportions will disturb the even distribution and dispersion of light within the diamond - this normally results in a loss of sparkle. However the GIA (Gemological Institute of America) does rate symmetry and there are seven ratings from Extremely Poor to Excellent. You should be fine with a symmetry rating ranging from Good to Excellent. It is common to have a price difference for better symmetry grading.