Diamond Fire, Brilliance and Scintillation Explained

What are diamond fire, brilliance and scintillation?

When people say a diamond 'sparkles', they are describing three separate things happening at once. Gemmologists call them brilliance, fire and scintillation, and together they make up what the trade calls light performance. Each comes from a different part of how light behaves inside the stone, and each can be strong or weak depending on how the diamond was cut.

• Brilliance is the white light returned to your eye from inside the diamond. A brilliant stone looks bright across its whole face, even in ordinary indoor light.
• Fire is the flashes of spectral colour (red, orange, blue and violet) you see as the stone moves. It happens because a diamond splits white light into its colours, an effect called dispersion.
• Scintillation is the sparkle: the on-off flashes of light and the pattern of bright and dark areas that change as the diamond, the light or your head moves.

None of the three depends much on colour or clarity. They are created almost entirely by cut: the angles, proportions, symmetry and polish of the facets. If you are new to the parts of a diamond, our guide to the table, crown, girdle and pavilion will help with the terms below, and the GemTrove jewellery glossary covers everything else.

Why does a diamond sparkle more than glass?

Three physical numbers explain it. The first is refractive index (RI), a measure of how much a material slows and bends light. Diamond's RI is about 2.42, compared with about 1.5 for ordinary glass. The higher the RI, the more light bends as it enters, and the more easily it is kept inside the stone.

The second is the critical angle: the angle beyond which light hitting an inside surface is reflected back rather than passing out. For diamond it is about 24.4°, one of the smallest of any gem, so light striking the pavilion facets from most directions bounces back up instead of leaking out the bottom. This mirror effect, called total internal reflection, is the source of brilliance.

The third is dispersion: how far a material spreads white light into its colours, like a prism. Diamond's dispersion is 0.044, enough to throw visible rainbow flashes but not so much that colour drowns out the white light. Here is how diamond compares with other materials you will see in jewellery:

A note on the word 'fire'. A fire opal is named for its orange-to-red body colour, not for dispersion, and any play-of-colour it shows comes from tiny silica spheres diffracting light. In diamonds, fire always means dispersion.

Brilliance vs fire vs scintillation: what is the difference?

The three trade off against each other. A cutter aiming for maximum white brightness with a large table and a low crown gives up some fire, because there is less sloping crown for light to split through on the way out. A steeper crown and smaller table push fire up but can cost some brightness. The round brilliants most gemmologists rate highly sit in a balance where all three are strong.

Scintillation has two parts worth separating. Sparkle (sometimes called flash) is the number and size of the flashes. Pattern is how the bright and dark areas are arranged. A good pattern needs contrast: a diamond with no dark areas at all looks washed out, like a glass of water, while one with too many looks dark and lifeless.

How do cut angles control diamond fire, brilliance and scintillation?

Light enters mostly through the table and crown, travels down to the pavilion, reflects off two pavilion facets and comes back out through the top. Whether it makes that trip depends on the angles. Our guide to crown and pavilion angles goes into the geometry; here is the short version for a round brilliant.

Angles work in pairs. A slightly shallower crown can be balanced by a slightly steeper pavilion, and the reverse, which is why GIA's cut grade for rounds judges the combination rather than each number alone. The classic reference is Marcel Tolkowsky's 1919 design, with about a 53% table, a 34.5° crown and a 40.75° pavilion. Modern top-grade rounds sit close to that, usually with a slightly larger table.

Smaller details matter too. A large open culet shows as a dark or see-through dot in the centre, which is why most modern rounds have none. How thick the girdle is affects weight and, in extreme cases, fisheye. And the way a cutter polishes the girdle area of a round, known as painting and digging out, can change the contrast under the crown and save weight, sometimes at the cost of light return.

Symmetry and polish finish the job. If facets are misaligned, light that should return through one facet leaves through another, and the pattern looks uneven or patchy.

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

These are the names for the ways light performance goes wrong. Most are visible to the naked eye once you know what to look for, and none of them shows up in the colour or clarity grade.

• Windowing: a pale, see-through patch in the centre where you can almost read through the stone. The pavilion is too shallow, so light passes straight out the back. It is most common in elongated fancy shapes; see windowing across diamond shapes.
• Fisheye: a whitish or grey ring just inside the table, which is the girdle reflected in the pavilion. It happens when the pavilion is very shallow, usually below about 40°, and gets worse with a thick girdle.
• Nailhead: a dark centre, as if the head of a nail were sitting under the table. The pavilion is too deep, often above about 42°, so light leaks out the sides of the pavilion instead of coming back up.
• Bow-tie: a dark band across the middle of an oval, pear, marquise or heart. A faint one that moves is normal; a solid one is not. Our bow-tie effect guide shows examples.
• Extinction: any dark area on the face of the diamond. Some is needed for contrast; too much, or dark areas that never change as the stone moves, means light is being lost or blocked.

All of these are forms of light leakage or obstruction. Leakage is light escaping through the pavilion; obstruction is light blocked by the viewer's own head, which is why a stone can look darker when you lean in close. Neither shows reliably in a still photo taken in perfect studio light, which is why the viewing tools further down exist.

Which diamond shape sparkles the most?

The round brilliant returns the most light, because its cone-shaped pavilion can be cut at close to the ideal angle all the way round. Fancy shapes have to compromise: an oval's pavilion angle changes from the ends to the sides, so some areas always perform better than others. That does not make fancies dull. It means proportions matter more, and the numbers that work are different for each shape.

Use these figures as a starting point, not a pass mark. Our shape guides go further: oval proportions, the pear proportions chart, cushion cut numbers, radiant cut ranges and emerald-cut proportions. The outline matters as well: the length-to-width ratio changes where a bow-tie falls and how much the ends sparkle.

Depth affects size as well as light. A deep fancy carries weight underneath where it adds no brightness, so it looks smaller and often darker than a well-cut stone of the same weight, one reason two diamonds of the same carat can look so different.

How do ASET and Idealscope images show diamond light performance?

These are simple viewers that show what your eye sees, with colour coding so you can judge light return from a still image.

• ASET (Angular Spectrum Evaluation Tool, developed by the American Gem Society) shows where the light reaching your eye comes from. Red is light from high above, the brightest source; green is light from lower angles; blue marks contrast, where the viewer's head blocks light; white or pale areas are leakage. A good round shows mostly red, an even symmetrical pattern, crisp blue arrows and very little white.
• Idealscope is a simpler version: red shows light return, white or pale patches show leakage, and black shows contrast. It is quick for spotting windows and leaky edges in fancy shapes.
• A hearts and arrows viewer shows optical symmetry rather than brightness: eight arrows face-up and eight hearts through the pavilion on a precisely cut round.

Our guide to reading ASET and Idealscope images walks through real examples, and how a hearts and arrows viewer works explains the symmetry check and what it can and cannot tell you.

Do lab-grown diamonds have the same fire and brilliance as natural diamonds?

Yes. A laboratory-grown diamond is the same material as a natural diamond, crystalline carbon with an RI of about 2.42 and dispersion of 0.044, so the optics are identical. Two stones cut to the same proportions look the same, whether one was mined or grown. Any difference you see comes from the cutting, not the origin. Lab-grown stones range from very precisely cut to cut mainly for size, so check proportions and an ASET image the same way you would for a natural diamond.

Lighting changes how any diamond looks. Under diffuse daylight or office light you mostly see brilliance. Under LED downlights, candles or direct sun, fire comes forward. Under the strong spotlights in most jewellery stores almost every diamond looks lively, which is why the window test in Raj's tip matters.

Cleanliness matters more than most people expect. A film of skin oil or hand cream on the pavilion stops light reflecting back up, and all three effects fade quickly. A soak in warm water with a drop of dishwashing liquid and a soft toothbrush usually brings the sparkle straight back.

How to judge a diamond's sparkle: step by step

1. Start with the cut grade. For rounds, look for GIA Excellent or an IGI Ideal or Excellent cut grade. For fancy shapes, where GIA gives no overall cut grade, look for Excellent or Very Good polish and symmetry.
2. Check the numbers. Compare table and depth with the ranges above and, for rounds, the crown and pavilion angles. A round pavilion of about 40.6°–41° with a crown of about 34°–35° is a good sign.
3. Look at the light map. Ask for an ASET or Idealscope image. You want mostly red, an even pattern and little white leakage.
4. Watch a 360° video. Look for sparkle as it turns, a bow-tie that comes and goes rather than staying dark, and no nailhead, fisheye or window.
5. Test two kinds of light. Diffuse daylight shows brilliance; a single downlight shows fire. A well-cut stone holds up in both.
6. Think about the setting. Most light enters through the top, so an open setting adds little, but heavy claws covering the crown and a dirty pavilion both reduce sparkle.