Round Brilliant Cut Diamonds: The 2026 Guide to the 100-Year Reign Hailee Steinfeld & Dua Lipa Just Reaffirmed
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"There is one diamond shape that has never relinquished its crown for over 100 years." That shape is the round brilliant cut.
In August 2025, when singer-songwriter Hailee Steinfeld debuted her engagement ring from NFL quarterback Josh Allen, the world saw a 3.5-4 carat round brilliant diamond set in a platinum pavé band, valued between $70,000 and $150,000. That same year, Dua Lipa's engagement ring from actor Callum Turner featured a 2.5-4 carat round brilliant set low in a chunky yellow gold semi-bezel: the absolute embodiment of "quiet luxury."
While marquise and oval "celebrity elongated cuts" dominate 2026 headlines, the latest bridal industry data still shows round brilliants at 26% of all engagement ring selections worldwide—the global #1. Why does this shape refuse to abdicate after a century? This complete guide unpacks the mathematical perfection Marcel Tolkowsky calculated in 1919, all the way through to how to choose one in 2026.
What Is the Round Brilliant Cut? The 1919 "Equation of Light"
The round brilliant cut features a circular outline with 57 facets (58 including the culet) precisely arranged to maximize light return. In 1919, the fourth-generation Antwerp diamond cutter Marcel Tolkowsky published his University of London thesis "Diamond Design," in which he derived—through optical and mathematical calculation—the exact angles and proportions that would make a diamond return light most beautifully to the eye.
His "Ideal Proportions" became:
- Crown angle: 34.5 degrees
- Pavilion angle: 40.75 degrees
- Table percentage: 53%
- Total depth: 59.3%
One hundred years later, no cut has been developed that surpasses these proportions. Approximately 90% of all diamonds in global circulation today trace their geometry back to Tolkowsky's equation.
The 57-Facet Architecture: 33 Crown + 24 Pavilion
The 57 facets are organized around the table (the flat top surface) with 33 facets on the crown (upper half) and 24 facets on the pavilion (lower half). Light enters through the table, reflects internally off the pavilion facets, and returns through the crown to the eye—a light path that produces brilliance (white light return), fire (rainbow dispersion), and scintillation (sparkle in motion) at levels no other cut can match.
5 Reasons Round Brilliants Still Win in 2026
1. Universal Permanence — The Ultimate "Safe Bet"
Marquise and pear shapes carry the inherent risk that "what's chosen today may look dated in ten years." The round brilliant has been the only cut that "has not looked dated for a single moment since its 1919 perfection." For an engagement ring meant to last a lifetime, no other shape embodies timelessness as completely.
2. Superior Resale Value
In the secondary diamond market, round brilliants exhibit the highest liquidity and the most stable value of any shape. Compared to fancy shapes (oval, pear, etc.) at the same quality grade, round brilliants typically command 10-15% higher resale prices.
3. Hailee Steinfeld & Dua Lipa Just Validated the 2026 Comeback
Among the celebrity engagements of 2025, two of Hollywood's most forward-looking style icons—Hailee Steinfeld and Dua Lipa—chose round brilliants. "Choosing tradition deliberately, in a media cycle drowning in fancy shapes"—this is the very definition of 2026's most nuanced "quiet luxury" statement.
4. Perfect Harmony with Every Setting
Solitaire, halo, pavé, bezel, three-stone, toi et moi—only the round brilliant integrates seamlessly with every setting style and metal combination. If you want maximum flexibility in setting choice, no other shape compares.
5. The Only Shape with Official Cut Grading
The GIA and other major gemological laboratories assign formal cut grades (Excellent, Very Good, etc.) only to round brilliants. Fancy shapes do not receive cut grades, making quality assessment more difficult and subjective.
3 Quality Markers That Separate Great Round Brilliants
1. Triple Excellent (3EX)
A diamond rated "Excellent" by GIA in all three categories—Cut, Polish, and Symmetry—earns the 3EX designation. The even rarer Hearts & Arrows (H&A) grade requires that, when viewed through a specialized scope, eight perfect hearts appear from the pavilion view and eight perfect arrows from the crown view—the pinnacle of optical symmetry.
2. Fluorescence
For D color diamonds, strong blue fluorescence can introduce a milky or hazy appearance. ADAMAS exclusively sources D color diamonds with "None" or "Faint" fluorescence to guarantee maximum transparency.
3. Girdle Thickness
"Thin to Medium" is ideal. Very Thin risks chipping; Very Thick wastes carat weight on internal mass that doesn't contribute to brilliance.
ADAMAS Round Brilliants — The Crown Cut, at the Highest Tier
Every round brilliant in the ADAMAS collection—from engagement rings and eternity bands to solitaire diamond necklaces—is sourced as D color, 3EX, Hearts & Arrows grade. Tolkowsky's century-old "equation of light" is paired with the highest-purity K18 gold and platinum to deliver brilliance designed to outlast generations.
Celebrity-driven fancy shapes have their moment. But "the cut that has not aged in 100 years"—that is, in the ADAMAS philosophy, the truest definition of luxury.
To see ADAMAS round brilliant diamonds in person, or to consult on detailed quality grading for your engagement ring or anniversary jewelry, please visit adamas-gold.jp. A dedicated jewelry consultant will guide you through every step of choosing a piece designed to last a lifetime—and beyond.
Conclusion — Choosing the Crown Is the Most Sophisticated Choice
The 57-facet "equation of light" Marcel Tolkowsky derived in 1919 remains the most beloved diamond cut on Earth a century later. As Hailee Steinfeld and Dua Lipa proved with their 2025 engagement choices, 2026 belongs to those sophisticated enough to "choose tradition deliberately." For something meant to last a lifetime, choose the round brilliant. One hundred years of history—and mathematics—have already proven why.