Anatomy of the Round Brilliant

57 Facets, One Optical System

A round brilliant diamond is an optical instrument with 57 or 58 precisely angled surfaces — facets — that work together to gather light, bend it, split it into spectral colors and return it to the viewer's eye. No single facet works on its own. Change one angle by half a degree and the performance of every neighboring facet shifts with it.

The architecture divides into three zones. The table — the large flat facet on top — gathers light from above and either reflects it back or channels it into the interior. The crown (8 bezel facets, 8 star facets and 16 upper-half facets) disperses light to create brightness, fire and the moving pattern of flashes we call scintillation. The pavilion (8 main facets, 16 lower-half facets and an optional culet) acts as a mirror system, sending light back up through the crown to the viewer's eye.

When every facet is placed with precision — when the angles and proportions form one coherent optical system rather than a collection of individually acceptable measurements — the result is a diamond that stands out. That coherence is what separates the work of a master cutter from factory production.

The Reference Point for Everything

The girdle diameter is the widest measurement of the diamond — the distance from one edge to the opposite edge, straight through the center. No round diamond is perfectly circular, so the minimum and maximum measurements are averaged. That average is the reference point against which every other proportion on the grading report is calculated.

When a report says "table 57%" or "depth 61.5%," those percentages are relative to this average girdle diameter. It is the denominator in every proportion on your report. Understand this number and every other measurement starts to make sense.

How Deep the Light Travels

Total depth is the distance from table to culet, expressed as a percentage of the average girdle diameter. It tells you how "tall" or "flat" the diamond is relative to its width.

Depth matters because it sets the path light travels inside the stone. Too shallow and light leaks out of the bottom — the diamond looks glassy and lifeless. Too deep and light escapes before it can return to the viewer — the diamond looks dark, often with a darker center. The right range balances light return with face-up spread, so the diamond looks its carat weight while returning as much light as possible.

For the Brian Gavin Signature standard, depth typically falls between 59.0% and 62.5%. But depth alone tells you very little — it is the sum of crown height, girdle thickness and pavilion depth, and those individual components matter far more than the total.

The Window Into the Diamond

Table size is the average of four measurements across the table facet, expressed as a percentage of girdle diameter. Think of it as the size of the window through which you view the diamond's interior.

A larger table gathers more light and can increase brightness — white light return. But it does so at the expense of crown facet area, which reduces fire — the spectral colors you see when the diamond moves. A smaller table leaves more crown area for fire but can reduce overall brightness.

This is one of the fundamental trade-offs in diamond cutting, and the best table size depends on its relationship with crown angle and pavilion angle. In my experience, tables between 54% and 58% paired with the right crown angle give the best balance of brightness and fire. GIA's "Excellent" cut grade allows tables up to about 62% — at which point, in my view, you are giving up fire for brightness gains most viewers will not notice.

The First Ancillary Angle

Star length is the length of the star facet expressed as a percentage of the distance from girdle to table. These eight small triangular facets sit between the table edge and the bezel facets, and their length controls how light moves from the table into the crown.

Star facets that are too short create a chunky, blocky look with large flashes but limited scintillation. Stars that are too long create a diffused "crushed ice" look where individual flashes are too small to register. A range of roughly 50% to 55% creates a balanced pattern of light and dark that gives the diamond visual life.

This is one of the "ancillary angles" that the Gavin Effect® patent addresses. Many cutters treat star length as an afterthought. In our work, it is a deliberate, calibrated choice that interacts with every other proportion on the stone.

The Prism

Crown angle is the angle between the bezel facet plane and the table plane — the average of eight measurements, reported to the nearest half degree. If the pavilion is the mirror, the crown is the prism. Its angle determines how effectively the diamond splits white light into spectral colors (fire) and how much light returns to the viewer (brightness).

Steeper crowns (higher angles) produce more fire but can reduce brightness if taken too far. Shallower crowns produce more brightness but give up the rainbow flashes that give a diamond its emotional impact. The interaction between crown angle and pavilion angle is the most important relationship in diamond proportions.

In our experience, crown angles between 34.0° and 35.0° — paired with the right pavilion angle — consistently produce the most optically balanced results. GIA's "Excellent" grade allows crown angles across a range of several degrees (roughly 31.5° to 36.5°), wide enough to include diamonds that look meaningfully different from one another. This is why a GIA "Excellent" cut grade is a starting point, not a destination.

Room for the Prism to Work

Crown height is the vertical distance from the table to the girdle edge, expressed as a percentage of the average girdle diameter. It follows from crown angle and table size — change either one and crown height changes with it.

Taller crowns give light more room to disperse into spectral colors before it exits. This is why our Signature Princess cuts have crown heights of about 15%, compared with the 10–12% typical of the industry by Brian Gavin's own measurements — the extra crown height acts as a more effective prism and delivers stronger fire.

For round brilliants, crown heights between about 14.5% and 16.5% typically indicate proportions that support strong fire. Below about 12%, you are often looking at a diamond cut for weight retention rather than optical performance.

The Mirror

Pavilion angle is the angle between the pavilion main facet plane and the table plane — the average of eight measurements, reported to the nearest 0.2 degrees. If any single number on a grading report tells you the most about a diamond's light performance, it is this one.

The pavilion acts as a mirror system. Light enters through the crown, strikes the pavilion facets, reflects to the opposite side, then travels back up through the crown to the viewer's eye. The angle must be right for total internal reflection — too shallow and light leaks out of the bottom; too steep and light is lost, creating dark areas under the table.

The difference between a pavilion angle of 40.6° and 41.0° cannot be seen on a proportion diagram, yet inside the diamond it visibly changes how the stone looks face-up. For our Signature diamonds, pavilion angles center around 40.6° to 40.8° — a demanding tolerance that takes careful, precise cutting to hold consistently.

How Much Room the Mirror Has

Pavilion depth is the distance from the culet to the girdle edge, expressed as a percentage of average girdle diameter. Like crown height, it follows from the angle — steeper pavilion angles produce deeper pavilions.

Pavilion depth matters because it determines how far light travels inside the mirror system before reflecting back. For rounds, pavilion depths of about 42.5% to 43.5% work well. Deeper pavilions hide carat weight where the viewer cannot see it; shallower pavilions cannot achieve proper total internal reflection.

The Second Ancillary Angle

Lower-half length is the length of the lower girdle facets (also called lower-half facets) expressed as a percentage of the girdle-to-culet distance. These 16 facets sit on the pavilion between the main facets, and their length strongly affects the diamond's scintillation pattern.

Shorter lower-halves create broader, bolder flashes — fewer sparkle events, each one larger. Longer lower-halves create more numerous, finer flashes — a busier, more active sparkle pattern. Lengths of roughly 75% to 80% tend to give the most visually appealing balance.

This is the other critical ancillary angle that many cutters neglect. Together with star length, lower-half length sets the diamond's "personality" — broad, slow-rolling flashes or fine, rapid scintillation — and shapes the Hearts & Arrows pattern. The Gavin Effect® specifications calibrate both ancillary angles as part of one optical system, not as independent variables.

Protection vs. Hidden Weight

The girdle is the narrow band at the diamond's widest point — the edge where crown meets pavilion. Its thickness is assessed both as a measured percentage of diameter (at the "hill" positions where bezel and main facets meet) and as a visual description (including the "valley" positions between them).

Girdle thickness is a balancing act. Too thin (especially "extremely thin") and the diamond is vulnerable to chipping during setting and wear — a durability concern responsible cutters avoid. Too thick and carat weight is buried in a band the viewer never sees — the diamond weighs more than it looks, which is poor value.

The descriptions range from extremely thin to extremely thick. For our purposes, "thin to medium" or "thin to slightly thick" is the best balance of durability and visual efficiency. Diamonds with "very thick" or "extremely thick" girdles are usually cut for weight retention at the buyer's expense.

Weight Tricks Every Buyer Should Know

Painting occurs when a cutter tilts the upper- or lower-half facets toward the neighboring bezel or main facets. The half facets become nearly parallel to their neighbors, blurring the boundary between them and changing the girdle's profile so that more weight is retained while the girdle description still looks acceptable on paper.

Digging out tilts the half facets the other way — away from the bezel or main facets — which also alters girdle thickness around the stone to retain carat weight.

The consequence is weaker brightness and a muddled pattern. Painted and dug-out facets do not handle light the way properly angled facets do: the contrast pattern blurs, brightness drops and the lively sparkle of a well-cut diamond is reduced. Both can occur on the crown, the pavilion or both, and GIA takes them into account in its cut grade.

This is one of the weight-retention tricks our Optical Balance philosophy rejects. A Brian Gavin Signature diamond will not show painting or digging out, because we cut for beauty, not for the scale.

The Point at the Bottom

The culet is the tiny facet (or point) at the very bottom of the pavilion where the main facets meet. Modern precision cutting usually produces diamonds with no culet facet — a "pointed" culet — or a very small one that is invisible to the naked eye.

Larger culets show as a visible dark circle when viewed face-up through the table — a hole in the diamond's light return pattern. For precision-cut diamonds, the culet should be "None" (pointed) or "Very Small." Anything beyond "Small" begins to affect the pattern and is best avoided.

Older diamonds — Old Mine Cuts and Old European Cuts — typically have large, open culets. That is part of their historical character, not a defect. But in a modern round brilliant, a visible culet signals outdated cutting or a lack of investment in precision finishing.

Why Proportions Alone Are Not Enough

Everything on this page can be measured, calculated and printed on a grading report. And every one of these measurements can fall within "Excellent" parameters while the diamond still underperforms. This is the central point many buyers miss: a grading report measures individual components; it does not show how they work together as an optical system.

Two diamonds can have identical proportions on paper and look different in person. The difference lies in the precision of facet placement — the accuracy of each facet's angle and alignment — which determines optical symmetry, Hearts & Arrows patterning and the overall coherence of the light performance. Tools such as ASET® make that difference visible.

This is what our cutting philosophy addresses. The Gavin Effect® is not just a set of proportions — it is a complete optical system in which every facet is calibrated to work with every other. The proportions create the framework; the precision of execution creates the result. Read more about the cutter behind it on Brian Gavin's page.

Continue Your Education

• The Gavin Effect® Round Brilliant — Brian's patented round brilliant
• Diamond Cutting Services — Our cutting philosophy and process
• Grading Labs & Certification — Understanding GIA reports
• Understanding Rare Diamonds — Complete education hub

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