What is brilliance in a diamond?

Brilliance is the white light reflected back to the eye from inside a diamond, produced by its cut geometry rather than its origin. Light enters through the table facet, bounces off the angled pavilion facets, and exits back through the table toward the viewer. In a well-cut lab-grown diamond, this path is precise: an IGI-graded Excellent cut maximizes the percentage of light returned, measurably outperforming poorly cut stones of any type. No other gemstone, including sapphire or ruby, achieves the same return of white light because no other material combines diamond's refractive index of 2.42 with its dispersion characteristics. For buyers, cut grade is the single biggest lever on brilliance, more than carat weight or color. A lab-grown diamond with an Excellent cut at 0.80ct will outshine a 1.00ct stone with a Good cut. Prioritize cut grade first when comparing stones.