You're shopping for a diamond and comparing specs. One site throws around "HCA scores," another talks about "ideal proportions," another mentions "AGS Light Performance." Which one is right? What are the actual golden proportions that matter?
This is your cheat sheet. Print it, bookmark it, share it. Here are the exact proportions that define an elite round brilliant diamond in 2024.
The Golden Proportions: Quick Reference
| Proportion | Ideal Range | Avoid Range | Why It Matters |
|---|---|---|---|
| Table % | 55.0–56.5% | <54%, >57% | Controls sparkle size |
| Depth % | 59.8–62.0% | <59%, >62.5% | Face-up size and balance |
| Crown Angle | 34.0–34.8° | <32.5°, >35.5° | Light return angles |
| Pavilion Angle | 40.8–41.0° | <40.6°, >41.1° | Critical light bounce angle |
| Polish | Excellent | Fair or Poor | Surface finish quality |
| Symmetry | Excellent | Fair or Poor | Facet alignment precision |
What These Proportions Mean
Table %: The ratio of the table facet (top flat surface) to the overall diameter. Too small (under 54%) and the stone looks "glassy" with excessive sparkle. Too large (over 57%) and it looks flat and dark. The sweet spot is 55–56%.
Depth %: The ratio of total height to diameter. This determines face-up size and overall light behavior. The ideal range of 59.8–62.0% ensures optimal proportions without sacrificing diameter. Avoid deep stones (over 62.5%) that look smaller than their weight.
Crown Angle: The angle of the upper facets relative to the horizontal. This interacts with pavilion angle to control how light bounces. Ideal is 34.0–34.8°. Too steep (over 35.5°) creates excessive sparkle but less light return. Too shallow (under 32.5°) creates dull appearance.
Pavilion Angle: The angle of the lower facets. This is the most critical proportion for light performance. Optimal is 40.8–41.0°. Above 41.1°, performance drops sharply due to light leakage. Below 40.6°, you get dark "nail heads" in the center.
Polish & Symmetry: Both should be "Excellent" to ensure the stone's surface is flawless and facets align perfectly. "Very Good" is acceptable but costs less. Anything below that is unacceptable.
Understanding Carat Weight and Size
These proportions also determine visible size. Two 1.00 carat diamonds with identical specs except depth will have different diameters:
- Optimal proportions (60.8% depth): ~6.50mm diameter
- Deep cut (63.0% depth): ~6.15mm diameter
That 0.35mm difference means the deep stone looks about 6% smaller, even though it weighs the same.
The HCA Score vs. CutGrade
You may have heard of the "Hoge Cut Advisor" (HCA) score. It's a popular free tool that grades diamonds 0–10 based on proportions. Scores under 2 are considered "ideal," 2–4 are "excellent," etc.
HCA is useful, but it's based on older research from the early 2000s. It doesn't account for modern optical modeling or the full range of light behavior. CutGrade builds on HCA principles but includes advanced light-tracing algorithms to score actual light performance, not just proportions.
A diamond that scores well on HCA will also score well on CutGrade, but CutGrade provides more precision and accounts for factors HCA misses (like fluorescence and specific angle interactions).
The 4 Cs vs. Proportions
Diamond grading includes the "4 Cs": Carat, Color, Clarity, and Cut. Cut includes proportions, but GIA's cut grading is broad. An "Excellent" cut can include poor proportions that still technically meet GIA standards.
This is why proportions matter more than the letter grade. Use the golden proportions above to identify truly excellent stones. Don't rely solely on GIA's Excellent label.
How to Use This Cheat Sheet When Shopping
1. Find a diamond you like. Request the full GIA report with measurements (not just the grade).
2. Check each proportion against the "Ideal Range" column above. If any fall into the "Avoid Range," pass on the stone.
3. Enter the specs into CutGrade's calculator to get a light performance score. Score 90+ means excellent, 85–89 means good, below 85 means pass.
4. Compare multiple diamonds using this system. You'll quickly identify which stones are truly elite.
The Three Tiers of Diamond Proportions
Elite (Top 10%): All proportions within ideal ranges. Table 55–56%, Depth 60–62%, Crown 34–34.8°, Pavilion 40.8–41.0°. These are the diamonds to buy if budget allows.
Excellent (Top 25%): Most proportions within ideal ranges, 1–2 slightly off but acceptable. Example: Table 56.5%, Pavilion 41.1°. Still great choices, slightly lower cost.
Good (Top 50%): Some proportions in acceptable but non-ideal range. Example: Table 57%, Depth 62.5%, Pavilion 41.2°. Visible trade-offs in sparkle or size. Use CutGrade to score first.
The Bottom Line
The golden proportions for 2024 are Table 55–56%, Depth 60–62%, Crown 34–34.8°, Pavilion 40.8–41.0°. These ranges ensure optimal light return, excellent sparkle, and fair face-up size. When shopping, don't settle for "GIA Excellent." Dig into the actual numbers. Use this cheat sheet and CutGrade to verify that the stone you're buying has true elite proportions.
Bookmark this article. Screenshot the proportions table. Use it every time you shop for a diamond. Elite proportions = Elite diamond.