Cut refers to the quality of a diamond’s angles, proportions, symmetry, brilliance, scintillation and fire. These factors directly impact a diamond’s power to sparkle, along with its overall aesthetic appeal. A diamond’s proportions determine how light performs when it enters the diamond. If light enters through the crown and leaves through the pavilion, the diamond will look dark and unattractive. The perfect cut and good polish make better use of the light, and will be bright, colorful, and scintillating. The GIA grades Diamond Cut on the scale of Excellent, Very Good, Good, Fair and Poor.
Excellent Cut: Only 3% of top diamonds are in this grade. It reflects most of the light that enters the diamond, representing a perfect and outstanding cut.
Very Good Cut: About 15% of diamonds fall into this grade. It reflects nearly as much light as the ideal cut, but slightly lower the price.
Good Cut: About 25% of diamonds fall into this grade. It reflects most of the light that enters.
Fair Cut: Roughly 35% of diamonds fall into this grade. The diamond is still of good quality, but the cut is average, and the amount of light reflected falls short of the Good grade.
Poor Cut: Diamonds of this grade do not reach average industry standards. The diamond is cut either too narrow or too wide, thus it loses light through the sides or the bottom.
Photo source: gia.edu