The proportion grade is a qualitative assessment of the overall proportions of a round brilliant cut diamond. It evaluates how well the diamond’s dimensions, angles, and facet arrangements are balanced to maximize brilliance, fire, and scintillation.
Why Proportions Matter
The way a diamond is cut significantly impacts its light performance. If the proportions are too shallow or too deep, light may escape through the sides or bottom of the diamond rather than reflecting back to the viewer, reducing its brilliance. The ideal proportion grade ensures that the diamond reflects and refracts light efficiently, creating maximum sparkle.
How Proportion Grade is Determined
For round brilliant cut diamonds, proportion grading is based on several key measurements:
Table Percentage – The size of the top facet (table) relative to the total width of the diamond.
Depth Percentage – The total depth of the diamond (from table to culet) relative to its width.
Crown Angle – The angle between the table and the crown facets, affecting light dispersion (fire).
Pavilion Angle – The angle between the pavilion facets and the girdle, influencing light return.
Girdle Thickness – Affects durability and light performance.
Culet Size – Affects light leakage and appearance.
Proportion Grades in Round Brilliant Diamonds
Grading laboratories, such as the GIA (Gemological Institute of America), categorize proportion grades into different levels:
Excellent (EX) – Ideal proportions for optimal brilliance, fire, and scintillation. Light is perfectly reflected, creating a highly desirable sparkle.
Very Good (VG) – Slight variations in proportions that still allow excellent light performance, with minimal impact on brilliance.
Good (G) – Noticeable proportion variations that may result in light leakage, reducing the overall brightness.
Fair (F) – Unbalanced proportions causing significant light loss, making the diamond appear dull or lifeless.
Poor (P) – Severe proportion flaws leading to excessive light leakage, drastically affecting brilliance and beauty.
Proportion Grading for Fancy-Shaped Diamonds
Unlike round brilliant cut diamonds, fancy-shaped diamonds (e.g., princess, cushion, oval, emerald) do not receive a proportion grade. This is because there is no universal standard for the best proportions in fancy shapes—each shape has unique visual and performance characteristics that cannot be judged by a single set of criteria. Instead, fancy shapes are evaluated based on symmetry, polish, and overall cut quality.
Ideal Proportions for Round Brilliant Diamonds
While exact measurements may vary slightly, the following proportions are considered ideal for maximizing brilliance in a round brilliant cut diamond:
Table Size: 53%–58% of the diamond’s total width
Total Depth: 59%–63%
Crown Angle: 34°–35°
Pavilion Angle: 40.6°–41°
Girdle Thickness: Thin to Medium
Culet: None or Very Small
The proportion grade is a critical factor in determining a round brilliant cut diamond’s overall light performance and beauty. Diamonds with Excellent or Very Good proportion grades offer superior brilliance, making them highly desirable. While fancy-shaped diamonds are not proportion graded, their individual cut quality should still be carefully evaluated to ensure maximum visual appeal.