Diamond Proportions

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.

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.

Round Brilliant Diamond Visual Tendencies

Click for PDF version.

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.


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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.


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Where to Find Proportions Data

There are eight facet groups on a round diamond: See Round Brilliant Diamond In Detail for a look at all of them.

Proportions Map

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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.

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