Round brilliant cut

"Round" is the outline. "Brilliant" is the facet pattern, mostly triangles and kites arranged to bounce light inside the stone and back out of the top. GIA counts 57 or 58 facets on a round brilliant, the 58th being the culet, a tiny flat facet at the bottom point. Gem-A explains the same count: 58 if a culet facet is present, otherwise 57.

GIA describes the round brilliant as returning the most brightness of any diamond cut. Brilliant cuts also scatter light across many small facets, which helps hide small inclusions and slight body colour. That is one reason the round is a common choice for a solitaire.

The modern proportions go back to Marcel Tolkowsky, who published Diamond Design in London in 1919. He worked out, with geometry, how the angles of the top (crown) and bottom (pavilion) decide how much light stays in the stone. His conclusion is a useful reference point, shown below as percentages of the stone's diameter. His own rounded summary was a table of one-half the diameter and a thickness of six-tenths of it. Tolkowsky left out the girdle and the culet, so a real stone measures a little deeper.

No. GIA's research found many combinations of proportions that give a round brilliant strong brightness and fire, not one perfect set. GIA spent 15 years building its cut grading system, using computer modelling of tens of thousands of proportion combinations and more than 70,000 human observations of real diamonds.

That is why you should not shop by a single percentage. Use the cut grade and the proportions on the grading report together, then look at the stone.

GIA grades cut only for standard round brilliants in the D to Z colour range, because they are the only shape with a standardized facet pattern. The grade runs Excellent, Very Good, Good, Fair, Poor and rests on three things: face-up appearance (brightness, fire, scintillation), design (weight ratio and durability) and craftsmanship (polish and symmetry). See diamond cut grade for how to read it.

All other shapes are called fancy shapes, and on a GIA report they get polish and symmetry grades but no overall cut grade. As of October 2026, trade press reported (June 2026) that GIA plans to add cut grades for oval, marquise and pear in 2027. Until then, a fancy shape is judged more by eye and by its measurements.

Choose it if you want maximum sparkle, a classic look and an easy comparison between stones. The trade-offs are real:

• Cost: Round brilliants tend to cost the most of all the shapes and cut styles, and a fancy shape can be a good way to save money.
• Apparent size: an oval has a larger surface area than a round of the same carat weight, so it looks bigger.
• Character: it is the expected choice. If you want something less expected, a cushion or emerald cut reads very differently.

The diamond shapes guide compares all of them side by side.

Start with the cut grade, then compare colour and clarity between stones that share it. Because a brilliant is better at hiding small inclusions, a slightly lower clarity grade may still look clean to the eye. Ask for the diameter in millimetres too, since two stones of the same weight can face up at different sizes.

If you want a round stone in a ring made for one person, a custom engagement ring starts from the stone and setting you choose, and the design studio lets you note a shape first. You can also browse engagement rings or visit a showroom to compare real stones.