GemDiagram Docs

Reference: Proportion Ratios

Proportion ratios describe the shape of a cut without referencing absolute size. A 1-carat Round Brilliant and a 10-carat Round Brilliant should have the same proportions — just different scale.

Common ratios

Ratio Formula Typical Range Notes
Depth / Width (D/W) Depth ÷ Width 58–65% How deep the stone sits. Round Brilliant is 59–65%. Shallow cuts (50–58%) look flat; deep cuts (>65%) look narrow.
Table / Width (T/W) Table diameter ÷ Width 50–58% The flat top as a percentage of the stone's width. Affects sparkle — too small looks sparse, too large looks dull.
Crown / Pavilion Crown depth ÷ Pavilion depth 0.7–1.0 The ratio of the top to the bottom. Most brilliant cuts have slightly deeper pavilions (ratio 0.7–0.9).
Girdle / Width Girdle thickness ÷ Width 1–4% The edge as a percentage of width. Thin girdles (<1%) are fragile; thick girdles (>4%) look heavy.
Pavilion Angle (measured in degrees, not a ratio) 30–45° Depends on the material's critical angle. Diamond (crit. 24.4°) can go as shallow as 30°; Ruby (crit. 23.1°) similarly. Pavilion must be steeper than critical angle.
Crown Angle (measured in degrees, not a ratio) 20–45° Typically shallower than pavilion. Controls how the crown catches light.

Round Brilliant "ideal" proportions

These are classical benchmarks (some debated among cutters):

  • Depth / Width: 59.3–61.0%
  • Table / Width: 53–56%
  • Crown angle: 34–35°
  • Pavilion angle: 40.75–41.0°
  • Girdle: 1.5–2.0%
  • Culet: Small (less than 1%) or absent

Slight variations (±2%) don't significantly impact light return, but they're useful guides.

Why proportions matter

  • Light return: Crown angles too shallow scatter light poorly; pavilion angles shallower than critical angle cause windowing.
  • Durability: Thin girdles are fragile; thick girdles protect the edge but add weight.
  • Aesthetics: Shallow stones (low D/W) look broad and flat; deep stones (high D/W) look narrow and sparkly.
  • Yield: Deeper designs waste more rough; shallower designs fit into rough better.

Using proportions in GemDiagram

The Stone Report (Inspect panel) shows your design's ratios. Compare them against these benchmarks:

  • If D/W is 50%, your stone is shallow — it'll look flat and bright.
  • If D/W is 68%, your stone is deep — it'll look concentrated and sparkly.
  • If T/W is 60%, your table is large — the look is open and spacious.
  • If T/W is 45%, your table is small — the look is intricate and complex.

No single "best" ratio exists — it depends on material, design intent, and personal taste. But these ranges help you understand what your design will look like before cutting.

Material-specific notes

  • Diamond: Can be shallower (pavilion 30°+) because its critical angle is lower. Can achieve higher light return with lower proportions.
  • Ruby, Sapphire: Similar critical angles to Diamond. Proportions are similar.
  • Quartz, Fluorite: Lower refractive index means shallower pavilions cause windowing more easily. Pavilion angles should be steeper.

See Refractive Index & Dispersion for critical angles by material.

Comparing cuts

Use these ratios to compare different designs:

  • Round Brilliant A: D/W 61%, T/W 54%, Crown 35°, Pavilion 40.8°
  • Round Brilliant B: D/W 59%, T/W 53%, Crown 35°, Pavilion 40.8°

Both are "ideal," but A is slightly deeper — A will look slightly more concentrated, B slightly broader.

References

For deeper dives, see: