The Diamond's Cut

The cut of a diamond determines how it reflects light, which is responsible for its sparkle or brilliance. A well-cut diamond is cut by a skilled professional to the best proportions possible so that light will be reflected from each of its mirrorlike facets and disperse through its top. Too shallow a cut will let light escape through a diamond's bottom, causing it to appear dull, while too deep a cut will allow light to be lost through a diamond's sides, making it appear dark.
Because a diamond with perfect color and clarity could nevertheless have poor brilliance if it is not well cut, many gemologists consider this to be the most important property to note when choosing a diamond. Diamonds can carry cut grades of Excellent, Ideal, Very Good, Good, or Fair.