The light of a diamond
Why can two diamonds of the same weight sparkle so differently? It depends on how they are cut. Change the proportions of the stone and see right away what happens to the light: this diamond is calculated ray by ray with the laws of optics.
Drag to rotate and tilt the stone
The perfect proportions of a round brilliant
- Table
- 56% of the diameter
- Crown angle
- 34.5°
- Pavilion angle
- 40.8°
- Crown height
- about 15%
- Pavilion depth
- about 43%
- Total depth
- about 61.5%
- Star facets
- about 50–55%
- Lower half facets
- about 75–80%
- Girdle
- thin to slightly thick
- Culet (bottom point)
- none or very small
- Symmetry and polish
- excellent
With these measurements the light that enters is reflected twice by the pavilion and returns to the eye. Seen from above, the stone shows a regular pattern of bright areas and eight dark arrows, the sign of a precise cut.
Inside the stone: the path of light
This is the same stone cut in half and seen from the side. The rays come down from above, enter through the table and the crown and meet the pavilion facets.
If the pavilion has the right slope, each ray strikes it at an angle oblique enough to be reflected completely, as if by a perfect mirror: it bounces off the other side and travels back up. If the pavilion is too shallow or too deep, the angle changes and the light escapes through the bottom.
Move the pavilion slider: you will see red rays appear at both extremes.
Why a diamond sparkles
Refraction
When light enters a diamond it slows down to less than half its speed and changes direction. Diamond has a refractive index of about 2.42, among the highest of natural gemstones: glass, by comparison, is around 1.5.
Total internal reflection
Such a high index has a precious consequence: if, inside the stone, a ray strikes a facet at more than about 24.4° from the perpendicular, it cannot get out and is reflected entirely. The brilliant cut is designed so that this happens on the pavilion.
Fire
White light is made of all colors, and diamond bends violet and blue a little more than red. As the light leaves the sloping crown facets, the colors separate: these are the colored flashes you see when the stone moves. This is called dispersion.
Scintillation
The play of bright and dark areas that changes with every movement: the facets light up and go dark in turn, partly because the viewer’s head casts a shadow on some of them. Let the stone rotate to see it.
The proportions that matter
As early as 1919 the engineer Marcel Tolkowsky calculated proportions very close to those still used today: a table of about 53%, a crown of 34.5° and a pavilion of 40.75°. Modern cuts usually have a slightly larger table. In the table, the ideal range is the one highlighted in green under the sliders; next to it are the wider limits within which GIA can award the “Excellent” grade. GIA, however, assesses the measurements together: even if each one is within its limits, an unfavorable combination can lower the grade.
| Measurement | Ideal range | GIA “Excellent” limits | If it is too low | If it is too high |
|---|---|---|---|---|
| Pavilion angle | 40.6° – 41.0° | 40.6° – 41.8° | The girdle appears reflected inside the table, the “fish-eye”; light starts to escape through the bottom | The center turns gray and dull, the “nailhead” effect |
| Crown angle | 34.0° – 35.0° | 31.5° – 36.5° | More white light but little fire | More fire, but the stone may look small for its weight |
| Table | 53% – 58% | 52% – 62% | More fire, less white light | More white light, less fire |
| Total depth | 59% – 62.5% | 57.5% – 63% | A stone that looks large but dull | Weight “hidden” at the bottom: the stone looks smaller |
How to read the cut on a grading report
The cut grade
For round brilliants the best-known laboratories, such as GIA and HRD Antwerp, give a cut grade: the highest is “Excellent”. It takes into account the proportions and how the stone returns light.
Symmetry and polish
These are two separate entries: they say how precise and well aligned the facets are and how smooth their surface is. Here too the best grade is “Excellent”.
The measurements
Many reports also list the table, angles and depth. This page gives you an idea of what those numbers mean.
The best way is still to see it in person
No screen truly conveys the scintillation of a diamond. In our shop on Ponte Vecchio we show you the stones in the right light and explain their grading report.
Write to us for informationWhere these numbers come from
The proportions on this page are not a matter of taste: they come from the optical calculation of the brilliant, begun more than a century ago and refined ever since. The starting point is diamond’s refractive index, about 2.42: from it you derive the angle beyond which light stays trapped inside the stone, about 24.4°, and therefore the slope the pavilion needs for light to be reflected twice and return upward, around 41°. The crown and table are then chosen to balance white light and fire. The ideal ranges are those on which the best cutters agree today; the “Excellent” limits are those laboratories use to grade the cut, set by comparing computer simulations with the observation of real stones. This simulator applies the same laws of optics to a brilliant with the measurements you choose: that is why, when you change the proportions, you see the same effects observed in real stones.
Illustrative simulation. The diamond is an ideal round brilliant with 57 facets, without inclusions and perfectly colorless, lit by jewelry-store lighting with several sources from above. The calculation follows the rays with refraction, total internal reflection, surface reflection and color dispersion: the values of the bars are for comparing cuts with each other, they are not laboratory measurements.