Brilliance

Brilliance is the total amount of white light a diamond returns to the eye when viewed face-up. It is one of three optical effects a cut diamond produces (alongside fire and scintillation) and is the effect most responsible for what a buyer perceives as overall "brightness" or "life."

Brilliance is a geometric outcome. Light enters through the crown, refracts at the air-diamond boundary, reflects off the pavilion mains via total internal reflection, and returns through the crown to the eye. The amount of light that successfully completes the round trip — versus the amount that escapes through the pavilion as leakage — is what brilliance measures. The geometry only works inside narrow proportion bands. Outside the bands light leaks; inside the bands light returns.

Pavilion angle is the most sensitive input to brilliance. A round with a pavilion angle outside 40.6° to 41.0° loses brilliance fastest of any single proportion deviation. Crown angle is the second input; table percentage is the third. Polish quality matters too — pavilion main facets with visible polish lines scatter light and reduce the proportion that returns cleanly to the eye.

The trade distinguishes brilliance from fire and scintillation. Fire is the spectral colour the stone disperses (the rainbow flashes); scintillation is the pattern of bright/dark flashes as the stone moves. Brilliance is the underlying total light return on top of which fire and scintillation play. A round with strong brilliance and weak fire reads as "bright but flat" — common in large-table commercial cuts. A round with strong fire but compromised brilliance reads as "sparkly but dim" — common in old European cuts at modern viewing distances.

AGS Light Performance grading measures brilliance directly via ray-trace and reports it on the AGS Ideal Report supplement. The brightness number is one of the four light-performance metrics graded 0 to 10; AGS Ideal-0 requires a 0 (top) on brightness. GIA does not publish brilliance as a separate metric — the GIA Excellent Cut grade folds it into the proportion-band assessment, which is why two GIA Excellent rounds can perform measurably differently on brilliance despite identical paper.

For a buyer comparing rounds, brilliance is the headline light performance metric. Crown/pavilion proportions in the AGS Ideal band predict it on paper; ASET imaging verifies it directly; hearts-and-arrows symmetry separates the top-tier executions from the merely-in-band proportions.