Cut refers to a diamond's proportions, symmetry and polish rather than its shape. A diamond's beauty generally depends more on cut than any other single factor. Diamond cut has three primary effects on appearance:1. Brilliance — the brightness created by the combination of all the white light reflections from the surface and the inside of a polished diamond; 2. Fire — the dispersion of light into the colors of the visible spectrum, seen as flashes of color; and 3. Scintillation — the flashes of light and dark, or sparkle, when a diamond or light source is moved.When a diamond is cut from a rough stone, there is always a balance between the diamond's optimal cut against the maximum yield of the carat weight. In short, there's a trade-off between getting more diamond and getting a more beautiful diamond. Because many customers are willing to pay more for a larger, fair-cut cut diamond than for a slightly smaller, well-cut diamond, there is pressure on the cutter to sacrifice appearance for weight. This is why the Cut grade is so important; it allows the purchaser to identify those stones that were cut Fair to Poor in an effort to gain carat weight.
DIAMOND CUT PROPORTIONS Diamond proportion refers to the relationship between the size, shape, and angle of each facet of a diamond. A wide range of combinations are possible, ultimately determining the diamond's interaction with light. When light strikes a diamond, approximately 20% immediately reflects off the surface (as glare). Of the 80% that enters, a portion will escape through the bottom of the diamond (where the observer cannot appreciate it). A well proportioned diamond will have each facet properly placed and angled so as to maximize the amount of light that reflects back out of the crown (top) of the diamond, to the eye of the observer. This reflected light is perceived as scintillation, fire and brilliance.