Diamond Fire – What exactly is it and how does it come about?

When we think about what makes a diamond beautiful, two words generally come to mind; fire and brilliance. Most people tend to think of them as one-in-the-same – basically the degree to which the diamond sparkles. But the two are actually distinct optical aspects. Brilliance can be thought of as brightness and scintillation; small flashes of bright white light. Fire is the dispersion of white light into rainbow colors as a result of refraction. It is therefore best to think of brilliance as white sparkles, and fire as color sparkles.
Ritani 1RZ7286 Knife-Edge Solitaire Engagement Ring

The Basics of Light

Visible light is part of a spectrum of electromagnetic energy that travels in waves. White light is made up of many wavelengths, each corresponding to a specific color. When light enters a transparent substance such as water, glass, or diamond, it slows down significantly. This slowing causes light to ‘bend’ or refract. Upon exiting the material back into air it speeds up again. Different wavelengths are bent at different rates and become separated or ‘dispersed’. It is this dispersion that allows for an observer to see the individual colors of a rainbow, colors refracted by a prism, and ‘fire’ from a diamond.
The video above is a comparison of two diamonds of different cut qualities, set side by side for the purposes of evaluating their ability to produce fire events in a lighting environment conducive to seeing this effect. The diamond on the left is a 0.90 H VS1 A CUT ABOVE® (AGS Ideal Hearts and Arrows) and on the right a 1.01 I VVS1 GIA Triple Excellent. (*details and diagnostic image package for both diamonds below)

The Magic of Fire

Experiencing fire in a diamond is similar to seeing a rainbow after a rain shower. It is always remarkable in its beauty and fleeting in its presence, which makes it all the more special. Like the colors of a rainbow, fire is light that has been “un-bundled” and separated into the individual colors of the spectrum by a prismatic effect. In a rainbow, the water vapor in the atmosphere acts as a prism as light travels through it. A faceted diamond also acts as a prism as light slows down upon entering and speeds up upon exiting the facets. The precise alignment of a diamond’s facets is critical in determining how probable it is to see fire events in a diamond.
Prism Rainbow
Nature Rainbow
When a light beam traveling at 186,000 miles per second goes from air and enters a transparent material such as glass or diamond, it is slowed down considerably. The more it is slowed the greater the material’s ability to refract the light. Diamond happens to have a very high refractive index. The higher the refractive index, the greater the potential for dispersion. While diamond therefore has a very high potential to exhibit fire, the quality of the cut is the key to whether that fire potential is realized.

The Role of Diamond Proportions

Through ray tracing it is possible to calculate the amount and location of dispersion of the light that the diamond is returning to the eye. The basic proportions must be within a relatively narrow range to provide a foundation for the generation of significant fire.
Ray Tracing

The Role of the Light Source

Our ability to observe fire also requires the proper lighting conditions. Like the aftermath of a rain shower that does not produce a rainbow, a well cut diamond in certain types of lighting environments will not produce fire.
Singular, direct light sources of small angular size are more likely to produce fire than large diffuse sources. Outdoors, in direct sunlight, a diamond can exhibit excellent fire.  However, if there is cloud cover, the light will be diffused. While this might be an optimal lighting environment for observing a diamond's brightness and contrast patterns, fire will be almost completely suppressed.
Indoor lighting with a number of individual lights of small angular size can produce a “fire friendly” environment to optimize our ability to observe fire. You may find this type of lighting in restaurants, stores, and even in elevators! By contrast, typical office lighting with long fluorescent tubes covered by plastic diffusers creates a lighting environment that suppresses fire. Fortunately, LED lighting is becoming ever more common and often features multiple small sources which are good for the observation of fire. The videos on this page were created in a controlled lighting environment with multiple LED light sources.

The Role of the Observer

Whether an observer perceives a colored sparkle (fire), depends on a number of factors including the viewing distance from the diamond. It also depends upon the observer’s pupil size as the fan of spectral color falls across their pupil and a specific color gets clipped from the fan.
As a dispersed beam of light exits the diamond, the different wavelengths have different trajectories that cause them to fan out even further with distance, increasing the chance that they can be clipped.
This is why you might see fire in a diamond on someone’s hand in a dimly lit restaurant from two or three tables away.
Prism Rainbow Observer
Eye Light Clipping
Spectral Fan Observer
Actual spectral fan from a diamond falling across an observer’s eye.
Red/yellow portion of spectrum missing the pupil which is clipping the green/blue portion.
Photo courtesy of GemConcepts Ltd.

The Role of Virtual Facets and Faceting Precision

The real work of light performance is done by ‘virtual facets’. These are areas of the diamond that reflect light back to the eye. The number of virtual facets is much greater than the number of physical facets, as a single facet can reflect light rays from multiple sources and directions. The larger the virtual facets are, the greater the probability that a fan will be large enough for a single wavelength to be clipped by an observer’s eye.
Virtual Facets
Virtual facets greatly outnumber physical facets
Therefore, the facet design must be one including some larger virtual facets. Precise facet alignment in 3 dimensions (optical symmetry) is critical in making sure the virtual facet sizes prescribed by the design are faithfully produced and not broken up in a chaotic and unpredictable collection of very small virtual facets. The craftsmanship of each facet is also critical to dispersion - they need to be polished to a perfectly flat mirror finish in order for the diamond to exhibit its full potential for fire.
The video above is the same video at the top of the page, but shown in incremental mode giving a view of the fire effects of extremely small variations in angles.

Laboratory Grading of Fire

The only scientific, peer reviewed evaluation of fire is contained within the light performance grading system developed by AGS Laboratories. A 3-D scan of the actual diamond is mathematically ray-traced and the effects of 32,000 rays are calculated. The diamond is graded for brightness, contrast, light leakage, and dispersion. The grade of ideal (0) is only awarded if the aspect has no significant deficits.
In addition to being able to generate ASET light maps for light return, light leakage, and contrast, the AGSL system also can produce fire maps. They indicate the areas of the diamond capable of producing fire.
Fire Maps


Fire is arguably the most magical aspect of diamond light performance and a key contributor to diamond beauty, even though it will not be exhibited in every lighting environment. The facet design and craftsmanship of the cut are the keys to producing a fiery diamond.
Ideal cut diamonds with top levels of optical precision, such as the A CUT ABOVE® in the videos above, excel in producing fire. In comparison to many GIA Triple Ex diamonds, the amount of additional fire produced by the highest cut quality is quite noticeable.
ACA Light Performance
AGS-104089831020 Light Performance Images
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GIA Light Performance
GIA2217469490 Light Performance Images
GIA Cert
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