Light Performance - Terms, Science & Dependency on 4C's

Diamond light performance is how well a diamond handles light: how much light it gives back to your eye, and in what form. It has four parts: brilliance (white light returned while the diamond is still), sparkle or scintillation (the flashes when it moves), fire (flashes of rainbow color) and light symmetry (how evenly that light spreads across the stone). Cut decides most of it. Color and carat weight hardly change the path light takes, and clarity matters only when inclusions are large.

Ever since I began selling diamond jewelry, I have faced abstract questions with no definitive answer. That makes sense: we are discussing diamonds, and diamonds make you feel rather than know. On the other hand, diamonds are measured with exceptionally advanced technology, so not being able to answer these questions puzzled me.

Questions I am often asked: Can I see the diamond under different lights? Does the diamond have a strong appearance? How strong is the sparkle? Why is one half of some diamonds dark and the other not? Does the diamond have "strong" fire?

What is diamond light performance?

In the previous article, I reviewed two ways to get a feel for a diamond and divided its features into nature-given (color, clarity, transparency) and human-given (cut, sparkle, brilliance, fire, light symmetry, polish and symmetry). I also defined light performance. Today I want to focus on each of its features: how we experience it, and what affects it.

Diamond light performance has four features: brilliance, sparkle, fire and light symmetry.

Feature	What you see	When you see it	What controls it
Brilliance	Bright white light coming back from the stone	Diamond and light still	Cut proportions (pavilion and crown angles)
Sparkle (scintillation)	White flashes, light against dark	Diamond, light or viewer moving	Facet pattern and contrast, crisp arrows
Fire	Flashes of rainbow color	Moving, best in sunlight or spotlights	Crown angles and dispersion
Light symmetry	Even brightness across the whole stone	Always; easiest to judge in an ASET image	Symmetry of the facets; large inclusions

What is diamond brilliance?

By definition, brilliance is "the intense white light that radiates from a diamond". Place your diamond on a surface and look at it. Does it look vivid and alive, or milky and dull? Is it radiating, or are its lights out? The answer should be immediate and intuitive. There is nothing to know when you look at your diamond; just call it as you see it.

Brilliance is how a diamond looks when it is not moving. While the diamond, the light source and the viewer are still, we can follow the path light takes inside the stone. Light enters through the table, reflects off a pavilion facet to the facet opposite, then travels back up and out through the top. That happens when the diamond is cut to good proportions. When it is cut too deep or too shallow, the light escapes through the sides or the bottom.

Neither the color nor the carat weight of the diamond affects this. Clarity can, if the inclusions are large enough to block light. In the clarity grades this series covers (SI2 and better), the visual impact, if any, is minor.

A diamond with healthy brilliance is mesmerizing, full of life, appealing and hypnotic. You are drawn to it, like a small sun. A diamond without brilliance is just a rock. By looking at the diamond through an ASET scope, we can see how much light it returns and from which directions.

What is diamond sparkle (scintillation)?

On to the king: sparkle, or scintillation if we want to get scientific. Sparkle is defined as "white flashes of light an observer sees when a diamond moves". It is the twinkle you get when light and dark areas trade places as the stone moves. In a round brilliant, it comes from good, solid arrows: the contrast of the arrows is what makes light twinkle into your eye. If you look at a diamond and see its arrows clearly, you may need your sunglasses.

What is diamond fire?

This is my personal favorite, what brings color and makes things interesting: fire. Who among us does not know the album cover of "The Dark Side of the Moon"? A white beam of light enters a triangular prism and leaves as a rainbow. A diamond does the same: as light leaves the stone, it bends and splits into its spectral colors. You see fire when the diamond, the viewer or the light source moves, as vivid flashes of color. It is most appealing in natural sunlight. Simply mesmerizing.

What is light symmetry?

Lastly, light symmetry. The cutting angles affect both the strength and the distribution of the effects above. A diamond can show outstanding light performance in one area while the light return is not equal: one part is bright (high light return) and another is dark, because the light is not bounced back or spread properly. Poor light symmetry may come from facets that are not symmetrical, or from a large inclusion. That is the answer to "why is one half of my diamond dark?"

Which of the 4Cs affect light performance?

• Cut: the big one. Proportions, symmetry and polish decide brilliance, sparkle, fire and light symmetry.
• Clarity: only when inclusions are large or dark enough to block light. Eye-clean stones perform like cleaner grades.
• Color: does not change the light path. A slightly warmer stone can show fire a little differently, but its brilliance comes from the cut.
• Carat: does not change light performance. A larger stone simply shows it on a bigger stage.

That is why shape and cut matter more than the certificate's color line when you want a lively stone. See how each shape handles light in do diamonds reflect or refract light?, and the difference between the effects in brilliance vs fire vs scintillation.

How is diamond light performance measured?

• ASET (Angular Spectrum Evaluation Tool): developed by the American Gem Society. It colors the light a diamond returns by the direction it comes from: red for the brightest light from high above, green for lower-angle light, blue for the contrast created by the viewer's own head. White or black areas show light leaking out.
• Ideal Scope: a simpler version that shows light leakage as pale areas.
• Hearts and arrows viewer: shows the arrows (from the top) and hearts (from the bottom) of a precisely cut round brilliant.
• Your eyes: look at the diamond still, then moving, then in sunlight. Brilliance, sparkle and fire, in that order.

To see light performance in real rings, compare a round, an oval and a radiant: the round diamond engagement rings, the natural oval diamond engagement rings such as Moonlit Sky, the Evening Light 2 carat radiant and the Luz 1.5 carat pear. For the calm flashes of a step cut, see emerald cut engagement rings.

Diamond light performance: FAQ

What is diamond light performance?

How well a diamond returns light to your eye. It is made of four features: brilliance, sparkle (scintillation), fire and light symmetry.

What affects a diamond's light performance the most?

The cut. Proportions, symmetry and polish decide how much light comes back. Color and carat weight barely change it, and clarity matters only when inclusions are large.

What is the difference between brilliance and fire?

Brilliance is the white light a diamond returns. Fire is the colored light: flashes of rainbow color that appear when light splits as it leaves the stone.

What does an ASET image show?

Where the light a diamond returns comes from. Red is the brightest light from high above, green is lower-angle light, and blue is contrast. White or black areas show light leaking out.

Why is one side of my diamond dark?

Usually poor light symmetry: facets cut at uneven angles, or a large inclusion, so light leaks out on one side instead of coming back. In ovals, pears and marquise cuts, a dark band across the middle is called a bow-tie.