The Cut Guide  ·  Reference Classic Shape  ·  Brilliant Family

Shape  ·  Face-Up View

Light Performance  ·  Live

Cut Reference  ·  No. 001

Round
Brilliant

Round Brilliant  ·  57–58 Facets

A century of mathematical refinement. The benchmark every other cut is measured against — and the only shape GIA grades for cut quality.

The round brilliant is the most analytically optimized cut in existence. Its 57–58 facets are arranged to maximize white light return, minimize leakage, and produce the strongest scintillation pattern of any diamond shape. It is the only cut assigned an official GIA cut grade — a distinction that reflects a century of scientific and commercial investment no other shape has received.

Origin

Marcel Tolkowsky published Diamond Design in 1919, deriving ideal crown and pavilion angles through geometric optics to balance brilliance against fire. His equations established the proportions the industry still builds on today. GIA introduced the formal cut grade in 2005 — applied only to round brilliants — which cemented its role as the universal benchmark. Every fancy shape that has followed has been engineered in its shadow.

On The Hand
Neutral
The circular outline has no elongating or widening effect — visually proportional on all hand shapes and finger lengths. No corners or points. The lowest-risk choice for any hand type.
57–58
Facets
#1
Popularity 2025
GIA
Only Graded Cut
1919
Tolkowsky
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Round Brilliant Cut thecutguide.com
Specifications
Table Percentage
53 – 58%
The industry sweet spot is 56%. Every point toward 58 adds brightness; toward 53 adds fire.
Depth Percentage
59 – 62.5%
Below 59% creates a fish-eye. Above 63% kills face-up size. This range is non-negotiable.
Crown Angle
34 – 35°
Tolkowsky's ideal is 34.5°. Controls the brilliance-to-fire balance — stay within 1° of target.
GIA Cut Grade
Excellent
The only cut GIA grades. Target Excellent — then verify proportions within the report. Very Good is acceptable; Good or below, avoid.
Polish / Symmetry
Exc / Exc
Non-negotiable. Both Excellent. Very Good acceptable on a tight budget — Good or below, walk away.
Light Performance
Brilliance96
Fire92
Scintillation90
Size per Carat76
Clarity Concealment88

These are the reference scores. All other cuts in this guide are benchmarked against round brilliant performance.

Budget
vs. Fancy Shapes
+15 to +20%
A structural premium from rough yield — rounds lose more material in cutting. Same carat, same grade, higher price.
Lab-Grown Round
−40 to −55%
Strong demand keeps lab-grown rounds at a higher premium than lab-grown fancy shapes. G, VS2, Exc/Exc is the target spec.
What Retailers Won't Tell You
⚠ "Excellent" Is a Wide Range
GIA Excellent covers tables from 52–62% and depths from 58–64%. A mediocre stone within the grade will underperform a precisely cut Very Good. The grade is not the cut. Use proportion data on the report — or an ASET image — to verify you're in the ideal subset.
⚠ Fluorescence Causes Haziness
Medium-to-Strong Blue fluorescence creates a milky, oily appearance in direct sunlight in approximately 5–10% of cases. It's on the GIA report but never mentioned at point of sale. Always view a fluorescent stone in natural light before purchasing.
The Cut Guide  ·  Assessment  ·  Round Brilliant
"The most studied cut in history — a century of refinement has left nothing to chance. The premium over fancy shapes is structural, not arbitrary. For buyers who want maximum light return and a stone that will never read as a trend, round brilliant is the correct answer."
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