Diamond Light Performance Explained: Brilliance, Leakage, ASET and Angles

Diamond light performance explained: what is it?

Light performance describes what a cut diamond does with the light that reaches it. Light enters through the top, bounces off the angled facets underneath and, in a well-cut stone, comes back out through the top towards your eye. In a poorly cut stone, some of that light escapes out the back or sides instead. That escaping light is called leakage, and it is the main reason a diamond can look glassy, grey or dull even when its colour and clarity grades are excellent.

Light performance is not a single grade on most reports. It is the combined result of the diamond's proportions (the angles and percentages of each part), how precisely the facets are aligned and how the stone is polished. If the parts of a diamond are new to you, our guide to the table, crown, girdle, pavilion and culet is a good starting point, and any unfamiliar word is in the GemTrove jewellery glossary.

The same physics applies to every diamond, natural or laboratory-grown, round or fancy shaped. What changes from shape to shape is how forgiving the outline is and which faults show up most.

How does light travel through a diamond?

Diamond has a refractive index (a measure of how strongly a material bends light) of about 2.42, one of the highest of any natural transparent material. Glass is about 1.5. That high figure gives diamond a small critical angle of about 24.4°. Any light striking an inside surface at more than 24.4° from straight on is reflected back inside rather than passing out. This is called total internal reflection, and it is what a cutter uses to turn the pavilion (the lower cone) into a set of mirrors.

For this to work, the pavilion facets have to sit at the right angle. In a round brilliant, light should hit one side of the pavilion, reflect across to the other side, and then rise up through the crown. If the pavilion is too shallow, light meets the first facet at too steep an angle and passes straight out the bottom. If it is too deep, light reflects once but then hits the second facet at too steep an angle and escapes out the side. Our guide to how crown and pavilion angles work together shows the geometry in more detail.

The crown (the upper part) matters too. Its angled facets bend light as it leaves the stone and split white light into spectral colours. Diamond's dispersion (how much it spreads light into colours) is about 0.044, well above most gems, which is why a good crown produces strong fire. At the very bottom, a large or open culet, the tiny facet at the pavilion tip can show as a small dark or see-through dot when you look down through the table.

What are brilliance, fire, scintillation and contrast?

Gemmologists break light performance into four visible effects. A diamond needs all four in balance; maximising one usually costs another.

• Brilliance: the total white light returned to your eye. It makes the stone look bright rather than grey.
• Fire: flashes of spectral colour, mostly from the crown. Most visible under spotlights and in candlelight.
• Scintillation: the on-off sparkle as the diamond, the light or your eye moves. It depends on the size and number of facets.
• Contrast: the pattern of dark areas, mostly reflections of your own head and body. Some contrast is essential; without it, sparkle has nothing to flash against.

A useful way to think about it: brilliance is the volume, fire is the colour, scintillation is the movement and contrast is the shadow that makes the rest visible. Our page on the difference between fire, brilliance and scintillation goes deeper into each one and how cutters trade them off.

What are nailhead, fisheye, windowing and extinction in a diamond?

Most light performance problems have names. Each one points to a specific proportion fault, which makes them easy to look for once you know them.

Nailhead and fisheye are the two extremes of pavilion depth, which is why the pavilion angle is the single most important number for light return. Windowing and fisheye are both forms of light leakage, where light exits the wrong way. Extinction is different: some darkness is normal and creates contrast, but a dark area that never lights up, whatever angle you view from, is lost brightness.

The bow-tie is a specific type of extinction in elongated shapes, explained in our guide to the bow-tie effect in ovals and pears. In rounds, cutters can also adjust the small facets near the edge, a practice called painting or digging out. It can change how the stone grades and how dark the outer ring looks; see how painting and digging out affect a round.

What are the best angles for diamond light performance in a round?

The round brilliant is the only shape where light performance is well mapped by research. In 1919 Marcel Tolkowsky published a design with a crown angle of about 34.5°, a pavilion angle of about 40.75° and a table of about 53%. Modern top-grade rounds sit very close to his angles, though tables are usually a little larger.

The crown and pavilion angles work as a pair. A slightly steeper crown can balance a slightly shallower pavilion, which is why two stones with different numbers can both perform well. GIA's cut grade for rounds (Excellent to Poor) takes these combinations into account, and a GIA Excellent round is a reasonable starting point, though the range inside Excellent is wide.

Girdle thickness affects how much weight sits in the edge rather than the face, covered in our guide to what girdle thickness means for size and safety. Precise facet alignment also matters: a stone can have good angles but misaligned facets that scatter light unevenly. That is the difference explained in symmetry grades versus optical symmetry, and the reason some buyers pay extra for hearts and arrows cut precision.

How are ASET and Idealscope images used to explain diamond light performance?

Numbers tell you what should happen. Reflector images show you what actually does happen. Both tools place the diamond under a coloured reflector and photograph it face-up, so each part of the stone is coloured by the direction its light comes from.

ASET (Angular Spectrum Evaluation Tool) was developed by the American Gem Society (AGS). It uses three colours:

• Red: light from high above the stone, between about 45° and 90°. This is the brightest light in most rooms, so you want plenty of red.
• Green: light from lower angles, between the horizon and about 45°. Still useful, but less intense.
• Blue: areas reflecting the viewer's head and body. This is contrast; it should be present and evenly spread.
• White or black: leakage, depending on the background. These areas are returning no useful light.

Idealscope is a simpler two-colour version. Red shows light return, black shows contrast, and pale or white areas show leakage. A good round looks evenly red with a crisp, symmetrical black pattern; a fisheye shows a pale ring and a nailhead shows a dark or washed-out centre.

Some laboratories go further and grade light performance directly. AGS Laboratories gave a Light Performance grade from 0 (Ideal) to 10 (the lab has since stopped issuing new reports, but older AGS reports still show it), and GCAL issues reports with measured brightness, fire and optical symmetry. Our page on light performance reports from GCAL and others compares what each one measures.

Does diamond light performance differ by shape?

Yes. A round brilliant has the same pavilion angle all the way around, so it can be cut close to optical perfection. Fancy shapes change width as you go around the outline, so the pavilion angle changes too, and some part of the stone will always be less than ideal. That is why fancy shapes show bow-ties, extinction and windowing more often, and why GIA gives no overall cut grade to them.

These are typical ranges, not rules. For each shape, see our detailed charts: oval diamond proportions, proportions that suit a pear, cushion cut depth and table ranges, radiant cut proportion targets and emerald cut proportions and windowing.

Step cuts such as the emerald work differently from brilliants. Their long parallel facets produce broad flashes of light and dark rather than pinpoint sparkle, so you judge them on clarity of the steps and an absence of windowing, not on fire. In elongated shapes, the outline also matters: our length-to-width ratio guide explains how a narrow or wide stone changes where light is lost.

Do lab-grown and natural diamonds perform the same in light?

Yes, when they are cut to the same proportions. Lab-grown diamonds are real diamond, with the same refractive index of about 2.42, the same dispersion and the same critical angle as natural diamonds. Light behaves identically in both. A well-cut lab-grown round and a well-cut natural round with matching angles will produce matching ASET images.

Where they can differ is in cutting priorities. Natural rough is costly, so some natural stones are cut deeper or with thicker girdles to keep weight, which can cost light return. Lab-grown rough is less costly, so cutters can more easily favour proportions, though plenty of lab-grown stones are still cut poorly for speed. Either way, check the angles and images rather than assuming.

Light performance also changes how big a stone looks. A diamond that leaks light or carries extra depth can look smaller and duller than a lighter, better cut one, which is the subject of why diamonds of the same carat look different and our guide to spread and face-up size.

How do you check a diamond's light performance before you buy?

1. Read the report. Note the cut grade (rounds), polish, symmetry, depth %, table %, crown and pavilion angles, girdle and culet, and whether the stone is natural or lab-grown.
2. Check the pavilion angle. For a round, about 40.6–41.0° is the target; below about 40° risks fisheye, above about 42° risks nailhead.
3. Check the crown angle against it. About 34–35° for a round; a steep pavilion with a steep crown is the most common route to a dark stone.
4. Ask for an ASET or Idealscope image. Look for even red, a symmetrical contrast pattern and no pale ring at the edges.
5. Watch a 360° video or view in person. Look for dark areas that stay dark, a fixed bow-tie or a see-through window in the centre.
6. Compare face-up size. Use the millimetre measurements, not only the carat weight, so a deep stone does not hide behind its weight.
7. View in different lighting. Daylight shows brilliance; spotlights or halogen show fire and scintillation.