How a diamond cut grade is set: proportions, polish and symmetry

1 in 30

GIA publishes tables to estimate the cut grade of a round brilliant. The tables give every combination of table size, crown angle and pavilion angle, each with its grade. There are 14,652 combinations.

We counted them. 478 are Excellent. That's about 1 in 30.

Most are Good (45%) or Fair (35%). And the Excellent ones aren't spread out. At a 53% table and a 34.5° crown, the pavilion must be between 40.6° and 41.4°. That window is less than one degree wide. A cutter who misses it by a few tenths of a degree drops a grade.

This page is about how that grade is put together. It also shows why one small flaw can pull down a stone that is perfect in every other way.

What goes into a cut grade

GIA's cut grading system for standard round brilliants (D to Z colors, 0.15 ct and larger) went onto its reports in January 2006. GIA built it in two steps:

• First, computer ray tracing of ideal diamonds, to predict how much light and fire they return.
• Then, more than 70,000 observations of real diamonds by people from the trade and the public. These showed GIA where to put the limits of each grade.

The grade of a diamond looks at seven components in three groups:

• Face-up appearance: brightness, fire and scintillation (in our words: brilliance, fire and scintillation).
• Design: the weight ratio and durability. The weight ratio tells if the stone has too much weight for its diameter. Durability tells if a very thin girdle puts it at risk.
• Craftsmanship: polish and symmetry.

An optical device measures the proportions: table, crown and pavilion angles, star and lower half lengths, girdle thickness and culet. It also measures any painting or digging of the facets. Graders judge polish, symmetry and the girdle and culet descriptions by eye.

The weakest part wins

Here's the rule that explains everything else: the system is deductive. The final grade is the lowest grade of the seven components. Six components Excellent and one Good make a Good stone.

Each proportion has its own limits. For Excellent:

• the table can be 52 to 62%
• the crown angle 31.5° to 36.5°
• the pavilion 40.6° to 41.8°
• the total depth 57.5 to 63%.

But the proportions also work together. Each value can be correct by itself, but the combination can still lower the grade. This happens most when two values are near the edge of their range.

Finish has its own ceiling:

In GIA's tests, trained observers saw no clear difference in the face-up look when polish or symmetry was one grade below the other cut components. So Very Good finish still allows an Excellent cut.

Try it yourself

1. Open Cut Grading Tool. It starts with the T73 as cut: 53%, 34.5°, 40.8°. Excellent.
2. Open More proportions and set Polish to Good. Read the grade and the Limited by line.
3. Put polish back. Set the Pavilion angle to 41.6°: by itself, that is inside the Excellent range. What happens? Now also set the Crown angle to 31.5°.
4. Rescue the Poor stone. Under Preset, choose Reference stone: Poor. Bring it to Excellent with as few changes as possible. After each change, read the Limited by line: it tells you what holds the stone back now.

What the Cut Grading Tool shows

The ceiling. With Good polish, the grade of the T73 stops at Very Good, but every proportion is still Excellent. The Limited by line tells you that.

The partnership. By itself, a 41.6° pavilion is inside the Excellent range. But with this crown and table, the combination is only Very Good. Make the crown shallower, 31.5°, and the pair is Excellent again. A shallower crown and a steeper pavilion balance each other. The grade treats the proportions as a team.

The rescue. The Poor reference stone has a 41.0° crown, a 44.0° pavilion, a thick 5.5% girdle, Good polish and Fair symmetry. The tool lists only what holds the stone at its current grade. So each fix shows you the next problem:

1. Poor, limited by a total depth of about 72%.
2. Fix the crown (34.5°) and pavilion (40.8°): Good, limited by symmetry Fair.
3. Raise polish and symmetry to Very Good: Very Good, limited by the girdle and the total depth.
4. Set Girdle thickness to 3.0% and Girdle description to Auto (it reads Medium from the thickness): Excellent.

That's six settings: crown, pavilion, polish, symmetry, girdle thickness and girdle description. The preset sets the girdle description to Thick by itself, so a thinner girdle alone isn't enough. You need each of the six settings. There's no shorter rescue.

Weight ratio. Step 3 is the "shrinking diamond" of a Poor cut, in grading terms. A thick girdle and a deep stone carry weight that a viewer can't see face up. The grade counts that against the stone, even when its pattern looks fine.

Our tool reads GIA's published estimation tables for education. It isn't GIA, it's not affiliated with GIA, and its result is not a GIA grade.

Key points

• GIA's cut grade for round brilliants combines measured proportions with visual checks of polish and symmetry.
• Seven components count: brilliance, fire, scintillation, weight ratio, durability, polish and symmetry.
• The grade is deductive: the weakest component sets it.
• The grade judges the proportions together. Two values that are correct alone can still lower the grade as a pair.
• Excellent needs polish and symmetry of Very Good or better. In GIA's tables, only about 1 in 30 combinations of table, crown and pavilion is Excellent.

Go further: in the Cut Grading Tool, set Chart view to Cut grade. Open More proportions and tick Show Very Good zone. Then change the Table size and watch the Excellent island move and get smaller.