Round Brilliant Cut

The Pinnacle of Diamond Cutting Science

The round brilliant cut represents the pinnacle of diamond cutting science. With its 58 precisely angled facets, this shape has been mathematically and empirically optimized to maximize brilliance, fire, and scintillation — the three components of a diamond's visual beauty.

Accounting for roughly three-quarters of all diamonds sold worldwide, the round brilliant's enduring popularity stems from both its light performance and its versatility. When cut to exacting standards, no other diamond shape returns light to the observer's eye as efficiently.

At RARE.DIAMONDS, we specialize in round brilliant diamonds cut to the highest standards of optical precision, including those featuring The Gavin Effect®. For the facet-by-facet detail, see Anatomy of the Round Brilliant.

The Peruzzi Cut (1700s)

The journey toward the modern round brilliant began in the early 1700s with Venetian diamond polisher Vincent Peruzzi, who is credited with increasing the number of crown facets from 17 to 33, creating what became known as the Peruzzi or “triple-cut” brilliant. This improved fire and brilliance compared with earlier cuts, though these stones appear dull by modern standards.

Because the bruting technique (cutting a diamond round) had not yet been developed, these early brilliants were rounded squares or rectangles in cross-section — what we now call Old Mine Cushion Cuts.

The Old European Cut (Late 1800s)

As diamond cutting tools advanced with steam-powered bruting machines, cutters could create truly round diamonds. The Old European Cut emerged, featuring 58 facets like modern brilliants but with different proportions: a smaller table, higher crown, larger culet, and broader facets resembling triangular blocks.

These cuts became immensely popular during the Art Deco era of the 1920s and dominated the American engagement ring market of that period.

Marcel Tolkowsky's Mathematical Revolution (1919)

The breakthrough came in 1919 when Belgian mathematician and diamond cutter Marcel Tolkowsky published his landmark work Diamond Design. Using calculations of light refraction and reflection, Tolkowsky set out the proportions he believed would maximize a diamond's brilliance and fire.

His formula specified precise angles for the crown (34.5°) and pavilion (40.75°), along with table and depth percentages. This marked the birth of the modern round brilliant cut — the first diamond cut based on optical science rather than trial and error.

Hearts & Arrows (1980s–1990s)

In the 1980s, Japanese cutters found that when round brilliants were cut with extraordinary precision and near-perfect optical symmetry, they displayed distinct patterns through specialized viewers: eight arrows face-up and eight hearts through the pavilion.

This standard of precision is achieved by only a very small share of round diamonds, and the pattern serves as visual evidence of exceptional cutting quality. Our Hearts & Arrows guide explains how the pattern is judged.

The Gavin Effect® (2016)

The most recent evolution came when fifth-generation master cutter Brian Gavin developed and patented a further refinement: The Gavin Effect® (U.S. Patent No. 10,405,618). The patent covers the optimization of ancillary angles to reduce the “green table effect” — low-level light leakage under the table facet — producing diamonds with stronger contrast, brightness, and light performance.

Facet Structure

Crown (33 facets):

• 1 table (the flat top facet)
• 8 bezel facets (kite-shaped facets surrounding the table)
• 8 star facets (triangular facets touching the table)
• 16 upper girdle facets (also called upper halves)

Pavilion (25 facets):

• 8 pavilion main facets (kite-shaped)
• 16 lower girdle facets (also called lower halves)
• 1 culet (optional small facet at the point; modern cuts typically have no culet or a “pointed” culet)

Critical Proportions

The interaction between all facets and their precise angles determines light performance. While Tolkowsky established foundational proportions, modern research has identified a range of optimal combinations:

• Table: 53–58%
• Depth: 59–62.5%
• Crown Angle: 34–35°
• Pavilion Angle: 40.6–41°
• Girdle: Thin to slightly thick
• Culet: None to very small

However, these primary angles tell only part of the story. The true mastery lies in optimizing the ancillary angles — the precise angles and lengths of star facets, upper girdle facets, and lower girdle facets — to create maximum optical performance.

U.S. Patent No. 10,405,618

Fifth-generation diamond cutter Brian Gavin holds this patent, which covers a method of maximizing light performance in the modern round brilliant cut. The Gavin Effect® represents years of research into optimizing the ancillary angles that most cutters overlook.

The Problem: Green Table Effect

Traditional super-ideal Hearts & Arrows diamonds, while exceptional, can still exhibit what's called the “green table effect” — areas of low-level light leakage visible under ASET (Angular Spectrum Evaluation Tool). This appears as green or white areas indicating light not being returned efficiently to the viewer's eye.

Most diamond cutters focus exclusively on the primary crown and pavilion angles. While important, these alone cannot eliminate all light leakage.

The Solution: Precision Ancillary Angles

Brian Gavin found that by precisely optimizing the ancillary angles — particularly the star facet length and lower girdle facet length — he could minimize or eliminate the green table effect. This creates diamonds with:

• Superior contrast patterns with deeper blacks and brighter whites
• The highest light performance grade available — AGS Ideal 0, the top result on the light performance scale AGS Laboratories built from reverse ray tracing and ASET analysis
• Exceptional scintillation with larger, more distinct sparkle flashes
• Enhanced fire with more pronounced spectral colors

Rather than quote a light return percentage — a figure no lab certifies — we let the graded result speak: the top of the scale, on a method built to measure exactly this.

Verifiable Performance

Every diamond cut with The Gavin Effect® is verified through multiple technologies:

• ASET imaging confirms minimal to no light leakage
• IdealScope shows superior light return patterns
• Hearts & Arrows viewers verify optical symmetry
• AGS Ideal® Report light performance grade — the ASET-based method developed by AGS Laboratories, issued by GIA since AGS closed at the end of 2022. The Gavin Effect® diamonds achieve the Ideal grade across all parameters. See Grading Labs & Certification

A Very Small Share of Rounds

Hearts & Arrows diamonds sit at the top of the round brilliant market — stones cut with such precision that near-perfect optical patterns emerge through specialized reflector scopes. Only a small fraction of round brilliants are cut to this standard, because it sacrifices weight for precision.

What Creates the Pattern

The hearts and arrows pattern is created by the overlapping of virtual facets and reflections within the diamond. When every facet is polished to exact angles with perfect indexing (alignment), these reflections organize into recognizable patterns:

• Eight symmetrical arrows visible face-up through the crown
• Eight symmetrical hearts visible through the pavilion

Any imprecision in facet angles, shape, or indexing causes distortions — hearts become irregular or look like “rabbit ears,” arrows become misshapen or misaligned. See the Hearts & Arrows guide for what to look for.

“It's All in the Hearts”

While many cutters focus on the arrows pattern (easier to achieve), true precision is revealed in the hearts. Well-formed hearts require exact pavilion main facet angles, precise lower girdle facet alignment, accurate culet placement, and consistent facet meet points.

As Brian Gavin states: “It's all in the hearts.” The hearts pattern is the ultimate test of a cutter's skill.

Beyond Visual Patterns

While Hearts & Arrows patterning indicates precision, it alone doesn't guarantee maximum light performance. A diamond can have excellent hearts and arrows but still exhibit light leakage if the ancillary angles aren't optimized — this is why The Gavin Effect® represents an advancement beyond traditional H&A cutting.

Brilliance (White Light Return)

Brilliance refers to the white light that reflects back to your eye. Superior round brilliants return the great majority of the light entering them, which creates extraordinary brightness and presence.

Brilliance depends on precise pavilion angles. Too shallow and light leaks out the pavilion. Too deep and light escapes through the sides. The optimal range is narrow — 40.6° to 41° — with interaction from crown angle and table size.

Fire (Spectral Colors)

Fire is the dispersion of white light into rainbow colors — the flashes of red, orange, yellow, green, blue, and violet you see when a diamond moves. Round brilliants are specifically designed to maximize this dispersion.

Fire depends on the crown angle and table size. Steeper crowns (34.5–35°) and smaller tables (53–56%) create more fire by increasing the path length light travels through the diamond.

Scintillation (Sparkle Pattern)

Scintillation is the pattern of bright and dark areas you see as the diamond, your head, or the light source moves. It's the “sparkle” that catches attention across a room. Superior scintillation shows distinct, crisp flashes rather than a hazy blur.

Scintillation depends on virtual facet size and contrast, controlled primarily by the ancillary angles — star facet length and lower girdle facet length. This is precisely what The Gavin Effect® optimizes.

How Light Performance Is Actually Measured

Light performance is not a single percentage. The method AGS Laboratories developed uses reverse ray tracing — tracing rays from the observer's eye back through the diamond — to assess brightness, fire, contrast, and light leakage together, and returns a graded result rather than a number. Ideal 0 is the top of that scale. The Gavin Effect® diamonds are cut to reach it, and the accompanying ASET image shows why: the leakage other cuts show under the table simply isn't there.

The Balance Point

The art of cutting exceptional round brilliants lies in balancing all three properties. Increasing fire might decrease brilliance. Maximizing brilliance might reduce scintillation. Master cutters like Brian Gavin understand these trade-offs and optimize proportions to achieve the ideal balance.

Colorless Diamonds (D-F)

The round brilliant is exceptionally effective at masking subtle body color. The intense light return and dispersed light paths help diamonds face up whiter than their actual color grade.

For rare colorless diamonds, round brilliants cut to superior proportions can make a G-color diamond appear closer to an E or F in many lighting conditions. This is why cut quality is paramount — it affects not just brilliance but perceived color.

Fancy Color Diamonds

The round brilliant's relationship with fancy colors is nuanced. While brilliant faceting maximizes white light return (which can dilute color), master cutters can adapt proportions to enhance color:

For Fancy Yellow: Slightly deeper pavilions and modified crown angles help saturate color while maintaining brilliance.

For Fancy Blue and Pink: The round brilliant can display these colors with real intensity. Blue Type IIb diamonds often show even color in round brilliant cuts because color distributes through the brilliant faceting.

However, many fancy color diamonds are cut in cushion, radiant, or other shapes that better retain color intensity — see Fancy Color Diamonds. The round brilliant is chosen for fancy colors where maintaining maximum brilliance while displaying color is the goal.

Why Cut Quality Affects Color

A poorly cut round brilliant shows more body color because light leaks out rather than reflecting back. When light escapes through the pavilion, you see into the diamond's body, making color more apparent. Superior cut quality masks color by reflecting light efficiently back to your eye.

Critical Angle and Total Internal Reflection

Diamond's refractive index (2.417) creates a critical angle of 24.4°. Light striking a pavilion facet at an angle beyond that critical angle undergoes total internal reflection — it bounces back into the diamond rather than escaping.

This is why pavilion angle is so critical. At 40.75° (Tolkowsky's ideal), light entering the table reflects off pavilion facets, bounces to opposite pavilion facets, then exits through the crown back to the observer's eye.

Crown Angle and Light Dispersion

The crown acts as a prism, separating white light into spectral colors. Steeper crown angles (34.5–35°) increase the path length light travels, creating more dispersion (fire). However, too steep and the diamond faces up dark because less light can enter.

The Table Size Balance

Smaller tables (53–56%) increase fire because more light passes through crown facets where dispersion occurs. Larger tables (57–60%) increase brilliance by allowing more direct light return but reduce fire. Modern super-ideal cuts typically use 55–57% tables for optimal balance.

Why Ancillary Angles Matter

While primary angles control the overall light path, ancillary angles determine:

• Virtual facet size (affecting scintillation pattern)
• Contrast distribution (balance of bright and dark areas)
• Light leakage zones (particularly under the table)

This is why The Gavin Effect®'s optimization of ancillary angles represents a genuine advancement — it addresses optical properties that primary angles alone cannot control.

Cutting Excellence

We focus on round brilliants cut to super-ideal standards:

• ✓ Hearts & Arrows precision verified through reflector scope imaging
• ✓ The Gavin Effect® diamonds (U.S. Patent No. 10,405,618)
• ✓ AGS Ideal 0 light performance grade, issued by GIA
• ✓ Verified light performance through ASET and IdealScope analysis

Our cutting services apply the same standards to stones clients already own.

Type Classification Priority

For colorless rounds, we prioritize Type IIa diamonds — the purest natural crystals, containing virtually no nitrogen. Type IIa diamonds show superior light transmission and often face up whiter than their color grade suggests.

For fancy color rounds, Type IIb blues receive particular emphasis for their exceptional saturation and rarity.

Five Generations of Cutting

Brian Gavin represents five generations of diamond cutting expertise dating to 1880s Amsterdam. This generational knowledge — passed from master to apprentice — cannot be replicated by modern production cutting houses. Read the full story.

When you acquire a round brilliant from RARE.DIAMONDS, you're acquiring a diamond cut with five generations of accumulated expertise in light performance optimization.

Each Shape Tells a Different Story

• Emerald Cut — Elegant step-cut with hall-of-mirrors effect; Brian Gavin's patented emerald cut maximizes light performance
• Cushion Cut — Romantic pillow shape with vintage appeal; patented Hearts & Arrows precision in a fancy shape
• Oval Cut — Elongated brilliant that appears larger for its carat weight
• Pear Cut — Elegant teardrop; exceptionally flattering in pendants and rings
• Radiant Cut — Mixed-cut combining brilliant fire with geometric outline; excellent for fancy colors
• Princess Cut — Modern square brilliant with exceptional scintillation
• Marquise Cut — Elongated brilliant with royal heritage; maximizes carat weight appearance
• Old Mine Cut — The antique cushion ancestor of the modern brilliant; candlelight fire and period character

All consultations are complimentary and without obligation. We build long-term relationships based on education, trust, and exceptional service.