Cut does not refer to the shape of the stone (e.g., round or square), but to how well the proportions, symmetry, and polish are executed to maximize light reflection. Every facet must sit at exactly the right angle and size. An 'Excellent' cut diamond catches the light perfectly and reflects it back with maximum brilliance, fire, and scintillation.

For round brilliants, GIA grades cut in five levels:

• Excellent: The best possible cut. Perfect proportions deliver maximum light throughput and the strongest brilliance. Sometimes called 'Ideal cut'.

• Very Good: Marginally below Excellent but virtually identical to the naked eye. Often an excellent cost-effective alternative.

• Good: Light reflects well but some 'leaks' out. Visible brilliance but not maximum.

• Fair: Noticeable light loss. The stone may look darker at the corners or center.

• Poor: Poor proportions produce a dark, lifeless stone – often a sign that carat weight was maximized at the expense of visual beauty.

A well-cut diamond creates three distinct visual effects:

• Brilliance: The white light reflected from the stone's interior and surface – the clear white glow that makes diamonds recognizable.

• Fire: The dispersion of white light into spectral colors (red, orange, yellow, green, blue) – the colorful 'flames' inside the diamond.

• Scintillation: The blinking pattern of light and dark fields as the diamond or the light source moves.

An Excellent cut round brilliant balances all three perfectly. This is precisely why cut carries the most weight of the 4Cs.