Diamonds can have cut grades of Ideal, Very Good, Good, Fair, and Poor. The cut, or proportions and angles, of a diamond determines how it reflects light. A diamond's reflective properties are referred to as "fire" or "brilliance." Too shallow a cut will let light escape through the sides of the diamond before it has a chance to reflect, making it appear watery or dull, while too deep a cut will allow light to be lost through the bottom of the diamond, making it appear dark, especially in the center.
Ideal: A diamond cut to extremely exacting proportions of depth, diameter, and angles, allowing for maximum brilliance (white light returning to the eye), dispersion (the play of colors you see in a diamond's reflections), and scintillation (sparkling flashes that are seen when the stone, light source, or the viewer moves).Diamonds with an Ideal cut grade are much rarer and more expensive than diamonds with a Very Good or Good cut grade.
Very Good:A diamond cut to exacting proportions, allowing it to reflect almost as much light as an Ideal cut would. Diamonds with a Very Good cut grade are brilliant diamonds with lower prices than Ideal cut.
Good: A diamond that might be slightly deeper or shallower, or have a slightly larger or smaller table facet, compared to an Ideal cut. Cutters may choose a Good cut in order to strike a balance between cut and carat weight retention. Diamonds with a Good cut grade reflect most of the light entering them and are an excellent value.
Fair: A diamond that reflects light well and costs less than a Good cut.