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Diamond Proportions: Modern Round Brilliant Cuts
The physics of light-behavior within a diamond are reliably consistent, which means proportions measurements can be used to draw certain conclusions about a diamond’s likely appearance.
For diamonds that deliver exceptional light performance and come with a comprehensive selection of light performance images and HD videos, we highly recommend A CUT ABOVE® from Whiteflash. Additionally, you may want to explore Astor by Blue Nile and True Hearts by James Allen for other top-tier options.
1. Descriptive Proportions Chart
Descriptions are general. Read these usage notes.
Be sure to use crown angle ° and pavilion angle ° data – not percentages (%).
Combinations on this chart range from predicted AGS 0 (Ideal) Light Performance to AGS 2 (Very Good).
Use the Holloway Cut Advisor (HCA) to verify predicted light return. The best HCA score is Excellent.
Request Ideal-Scope or ASET images for diamonds you are considering and post them on our forum for help.
Warning: This chart does not account for factors which may interfere with light transmission like undisclosed haze, clouds, etc. Read our page on Diamond Clarity for this information, along with the useful PriceScope eye-clean definition.
Limitation: Proportions data cannot be used to predict 3D optical precision, aka Hearts & Arrows.
*Confirmation: Proof trumps numbers. Request an Ideal Scope or ASET image of the diamond to verify cutting consistency and predicted performance.
Want help? Post information about any diamond you’re considering on our Diamond Discussion Forum for free guidance from dozens of experienced jewelry lovers and experts.
If you're looking for diamonds with strict proportions and the best light performance imaging consider Whiteflash A CUT ABOVE diamonds - these gems represent the pinnacle of diamond cut quality.
2. Member Recommended
Members of the PriceScope community have historically recommended a narrow range of “foolproof” proportions which receive high marks in all cut assessment systems: HCA Excellent, AGS Ideal or Excellent and GIA Excellent.
Largely contained in the “All Around” category above, diamonds with these numbers have robust light return and are predicted to have balanced brightness, fire and contrast.
Member Recommended Proportions*
Table: 55 – 58 %
Crown Angle: 34 – 35 °
Pavilion Angle: 40.6 – 40.9 °
Lower Halves: 75 – 80 %
Depth: 60 – 62.5 %
Girdle: Thin-Medium or Medium-SlightlyThick
–
If using GIA or GCAL report data some numbers are rounded up or down, therefore:
When pavilion angle is 40.8 ° any crown angle above is okay
When pavilion angle is 40.6 ° lean toward crown angle 34.5 – 35.0 °
When pavilion angle is 41.0 ° lean toward crown angle 34.0-34.5 °
When pavilion angle is 40.8 ° and crown angle is 35 ° lower halves need to be 80% *
Warning: These recommendations do not account for factors which may interfere with light transmission like undisclosed haze, clouds, etc. Read our page on Diamond Clarity for this information, along with the useful PriceScope eye-clean definition.
Limitation: Proportions data cannot be used to predict 3D optical precision, aka Hearts & Arrows.
*Confirmation: Proof trumps numbers. Request an Ideal Scope or ASET image of the diamond to verify cutting consistency and predicted performance.
Want help? Post information about any diamond you’re considering on our Diamond Discussion Forum for free guidance from dozens of experienced jewelry lovers and experts.
If you’re looking to buy loose diamonds online, use our special tools to reveal the best cut quality diamonds! Start your diamond search and choose from over a million loose diamonds for sale. Use our filters to find either natural or lab diamonds, as well as fancy color diamonds.
What Numbers are Rounded?
Updated: July 24, 2021
With the exception of AGS Laboratories, nearly all data points on grading reports are averaged and additionally rounded.
| Major Proportions | AGS-IGI report to: | GIA-GCAL report to: | Average of: |
| Table Facet | AGS 0.1%, IGI 0.5% | nearest 1% | 1 measurement |
| Crown Angle | 0.1 degree | nearest 0.5 | 8 measurements |
| Pavilion Angle | 0.1 degree | nearest 0.2 | 8 measurements |
| Girdle Thickness | Description | GIA 0.5%, GCAL 0.1% | variable |
| Culet | Description | Description | 1 measurement |
| Minor Proportions | AGS reports to: | Others report to: | Average of: |
| Star Length | 1% | nearest 5% (IGI NR) | 8 measurements |
| Lower Half Length | 1% | nearest 5% (IGI NR) | 16 measurements |
| Upper Half Length | Not reported | Not reported | 16 measurements |
| Calculations | AGS reports to: | Others report to: | Average of: |
| Total Depth % | 0.1% | 0.1% | Not averaged |
| Crown Height % | 0.1% | nearest 0.5% | Not averaged |
| Pavilion Depth % | 0.1% | nearest 0.5% | Not averaged |
Learn more about brightness, leakage, contrast and scintillation on our page Diamond Performance Explained.
Get fast answers to any question: Ask our community of unbiased independent helpers.
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