Which proportions play the biggest role in determining a round brilliant diamond's brightness?

• A. Table percentage, girdle thickness, depth
• B. Table percentage, crown angle, pavilion angle
• C. Table percentage, crown angle, and pavilion angle play the biggest role in determining a round brilliant diamond's brightness.
• D. Crown angle, pavilion angle, culet size

Answer: (C)

Light performance in a round brilliant is driven by how the light is guided back to the viewer, which hinges mainly on the table width and the crown and pavilion angles. The table percentage sets how much light can enter and how it exits the diamond, influencing the size of the light-return pattern and overall brilliance. The crown angle determines how light entering the crown facets is reflected upward toward the viewer, so the right angle helps maximize light return rather than letting it escape through the crown or girdle. The pavilion angle governs how light reflects inside the lower portion and back up through the crown; when this angle is balanced with the crown and table, more light is directed back to the observer’s eye, boosting brightness.

These three proportions work together to optimize the path of light inside the diamond. Girdle thickness, depth, and culet size can affect other aspects like durability, overall proportions, or potential light leakage in edge cases, but they don’t influence brightness as directly as the combination of table percentage with crown and pavilion angles. That’s why the trio of table width, crown angle, and pavilion angle best determines a round brilliant’s brightness.