Elements of Light Return - Fire

Some content on this page are contributed by Good Old Gold, with thanks.

What is Fire?

AGS defines fire/dispersion as The separation of white light into spectral colors. A diamond's fire is therefore the return of coloured light to the observer.

We know that in optics, when white light enters a prism, it is broken into its constituent colours. This is because each coloured light has different wavelengths. So, when light enters the prism at an angle, each constituent coloured light bends at a different angle. As a result, we observe coloured light exiting the prism.

Similarly, when a ray of light strikes a diamond, part of it is reflected. This is called External Reflection. Part of it, however, enters the diamond and bends because the diamond is more optically dense compared to air. This is called Refraction.

Assuming that the diamond is cut well, the light entering the diamond will reflect off the internal surfaces and returns to the observer. Some of these rays of light exit as a rays of white light. Depending on the angle of entrance and exit, some of these rays of light exit as spectrums of coloured light. This is called Dispersion.

Once the light has dispersed, you will be able to see the constituent coloured light.

Take a look at the diamond on the left. We can see yellow, green, pink, orange, and purple light.

Most people say (emphatically) that they would like their diamonds to be fiery. So let us examine the conditions under which fire would be most prominent.

The strength of fire observed within diamond depends primarily upon 2 factors:

• The strength of the light source in which the diamond is being observed under and

• The ability of the facets to function as reflectors.

We will examine each of these in greater detail below.

Lighting and Fire

Soft dispersion observed in bright natural ambient daylight	Strong dispersion observed in direct spotlighting

A general rule of thumb with diamonds is the stronger the lighting conditions the more the diamond should be functioning in its role as a prism (and therefore produce more fire). The sun, being one of the strongest light sources possible will produce strong flares of dispersion as it hits a well cut diamond. 

While strong fire is a highly desirable optical feature to observe in diamonds, the rest of the diamond, which is not reflecting back those flares will generally appear very dark or even silvery looking. Many people, when viewing their diamond for the first time in direct light conditions often become concerned because of the darkness that appears and oftentimes confuse this as being inferior in color when that in fact is not the case.  Even a "D" colored diamond will appear dark in direct lighting.  In a well cut stone however, you will see beautiful flashes of coloured light amidst the darker facets.

In bright ambient light (such as natural daylight on a cloudy day, under a shade) it is very possible to see a soft dispersion playing amongst the facets with a nice mix of white light akin to the model on the left in the illustration above. Note the soft dispersion of colors coming out of the diamond with a nice mix of white light return as well.

The graphic to the right depicts direct spotlight that is strong enough to produce strong dispersion.  Look at how the diamond glows with coloured light!

So, while people would like to see strong fire all the time, it is not possible as it is dependent on lighting. Anyway, I think this characteristic makes the diamond more interesting as it behaves differently as you move from room to room.

Proportion and Fire

Before a diamond can disperse light as fire, it must be able to perform its basic task of reflecting light back to the observer. Naturally, a diamond must be cut with good light return before it has strong fire. So, of course an ideal cut diamond will have strong fire under direct spotlighting.

Diamond Cut	Ideal Cut	Steep Crown /Deep Pavilion	Shallow Crown /Shallow Pavilion
Static Image
Dynamic Image
Result	strong fire	weaker fire due to leakage	strong fire in shallow combo

The diamond with the steep crown and deep pavilion used in the example above (second diamond) is graded as GIA Excellent. GIA relies more heavily on the office lighting view during their assessment of brightness (with contrast), fire and scintillation. This diamond will appear bright under office lighting. So, overall the cut is considered good. Unfortunately, if you are looking for a diamond with excellent fire, you will be disappointed with this diamond if you relied on the GIA cut grading when you made your purchasing decision. This points out the importance of assessing the diamond visually before making up your mind.

The third diamond has a shallow crown and shallow pavilion. It gets an AGS "2" cut grade for performance and a GIA Very Good (their 2nd grade). If you take careful note in the graphics above, the shallow/shallow has fire/dispersion on par with the Ideal in the first column. This tells us a few things.  If you wear your diamond mainly in strong direct light, you could deliberately find a diamond with steep/deep combination and get a nice fiery stone with strong dispersion for a cheaper price.  However the cheaper price will also dictate perhaps a not so pleasant appearance in other lighting conditions.  I believe most people move around different lighting conditions. So, I do not think that it is wise to choose a diamond that performs only in strong direct lighting. We also learn that it would be foolish to choose your diamond under the strong lighting in jewellery stores because you will be disappointed once you take the diamond into other lighting conditions.