The round brilliant is the most common diamond shape, with 57 facets, or 58 when the cutter adds a tiny culet facet at the bottom point. Its proportions are designed to send as much light as possible back out through the top. Small changes to the crown or pavilion angle can make one round diamond look bright and lively and another dull. This page covers each part of the cut, the proportions that work, and how labs grade them.
Parts of a round brilliant
A round brilliant has three main zones. The crown sits above the girdle, the girdle is the thin edge at the widest point, and the pavilion is the cone below.
| Part | Facets | What it does |
|---|---|---|
| Table | 1 | Largest facet on top; main window into the stone |
| Star facets | 8 | Small triangles around the table |
| Bezel (kite) facets | 8 | Large crown facets between table and girdle |
| Upper girdle facets | 16 | Triangles along the top edge of the girdle |
| Lower girdle facets | 16 | Long triangles along the bottom edge of the girdle |
| Pavilion main facets | 8 | Large facets running from girdle to culet |
| Culet | 0 or 1 | Bottom point, either left pointed or polished as a tiny facet |
That makes 33 facets on the crown and 24 or 25 on the pavilion. Some cutters also facet the girdle, which adds dozens of small facets that are not counted in the 57 or 58.
How the cut returns light
Diamond has a refractive index of about 2.42, which is very high. Light entering through the crown bends sharply, travels to the pavilion facets and strikes them at a steep angle. When the pavilion angle is right, that light reflects from facet to facet and exits back through the crown toward the viewer. This is total internal reflection, covered on the diamond optical properties page.
Three visual effects come from a well-cut round brilliant:
- Brightness (brilliance): white light reflected back to the eye
- Fire (dispersion): white light split into spectral colors, which diamond does at a rate of about 0.044
- Scintillation: the pattern of light and dark flashes as the stone, viewer or light moves
A pavilion that is too shallow lets light pass straight through the bottom, which can leave a dull ring called a fisheye, where the girdle reflects in the table. A pavilion that is too deep sends light out the sides and leaves a dark center, sometimes called a nailhead.
The crown
The crown angle is measured between the bezel facets and the girdle plane. Crown height is the distance from girdle to table as a percentage of diameter. A steeper crown tends to produce more fire. A flatter crown tends to produce more white brightness but less color.
The table percentage is the table's width divided by the stone's diameter. Larger tables make a stone look a bit bigger and brighter face-up but can reduce fire. Most well-cut round brilliants have tables somewhere in the mid-50s to around 60 percent.
The pavilion
The pavilion angle is the most important single figure on the stone. Well-cut round brilliants usually fall around 40.6 to 41 degrees. Pavilion depth, the distance from girdle to culet as a percentage of diameter, follows from the angle and usually sits around 43 percent.
Crown and pavilion angles work together. A slightly steeper crown can balance a slightly shallower pavilion and the reverse. That is why labs grade the full combination instead of checking each number alone.
The girdle
The girdle protects the edge from chipping. Labs describe its thickness on a scale of extremely thin, very thin, thin, medium, slightly thick, thick, very thick and extremely thick. An extremely thin girdle is at risk of chips. A very thick girdle adds weight that does not show face-up and makes the stone look smaller for its carat weight.
Girdles can be left bruted (slightly frosted), polished or faceted. Labs can laser-inscribe a report number on the girdle; the diamond certificates page explains how to check it.
The culet
The culet is the bottom tip. Modern round brilliants usually have no culet (a point) or a very small one. Labs grade culet size from none through very small, small, medium, slightly large, large, very large and extremely large. A large culet shows as a dark circle through the table, a look typical of old European cuts.
Tolkowsky's ideal
In 1919 Marcel Tolkowsky published a set of proportions for the round brilliant based on how light travels through diamond. His figures included a pavilion angle of about 40.75 degrees, a crown angle of about 34.5 degrees and a table of about 53 percent. He assumed a knife-edge girdle and no culet, which real stones cannot have, so cutters adjusted the design for real stones.
Tolkowsky's work set the direction for the modern cut, and "Tolkowsky ideal" still appears in trade language. Modern cut grading uses computer modeling and observation trials and accepts a wider range of combinations than his single set. The history of diamond cutting traces how the brilliant reached its current form.
Cut grades
GIA introduced a cut grade for standard round brilliants in 2006, on a five-step scale of Excellent, Very Good, Good, Fair and Poor. It considers brightness, fire and scintillation as predicted from the proportions, and also weight ratio (whether the stone carries extra weight for its diameter), durability (girdle thickness), polish and symmetry.
The AGS used a 0 to 10 scale, with 0 Ideal as the top grade, and added light performance analysis. The AGS diamond report page covers that system. Both labs grade polish and symmetry separately as well.
Hearts and arrows
A round brilliant with very precise symmetry shows a pattern of eight arrows when viewed through the table with a special viewer, and eight hearts when viewed from the pavilion side. The effect depends on the facets lining up evenly in size, angle and position.
Hearts and arrows patterns show optical symmetry. They do not prove ideal proportions on their own, because a stone can show the pattern with angles outside the best range. Buyers who want this cut usually look for a top cut grade plus excellent polish and symmetry on the report.
How the round compares to other shapes
The round brilliant generally loses more rough weight in cutting than fancy shapes, which is one reason rounds cost more per carat than comparable pears, ovals or princesses. See how diamonds are cut for the yield trade-off and patented and branded diamond cuts for other designs.
Frequently asked questions
How many facets does a round brilliant diamond have?
A standard round brilliant has 57 facets, or 58 if the cutter polishes a culet facet at the bottom point. Faceted girdles add more, but they are not counted.
What is the best pavilion angle for a round diamond?
Well-cut round brilliants usually have pavilion angles around 40.6 to 41 degrees. The right angle also depends on the crown angle, so labs judge the combination.
Does hearts and arrows mean a diamond is ideal cut?
No. Hearts and arrows shows precise optical symmetry. Proportions still need to fall in a good range for the stone to return light well.
Why is a round diamond more expensive than other shapes?
Cutting a round from rough usually removes more weight than cutting a fancy shape, and demand for rounds is high. Both push up the price per carat.