Light performance can be broken down into brilliance (brightness), fire, and scintillation (sparkle). The unique proportions and the optical symmetry of a diamond will give each diamond a slightly different light performance profile.
In part one, I explained why light return is important and why you want to avoid light leakage in a diamond. I also showed you that the direction from where light is being returned has a big effect on the strength of that light return. But most importantly, I touched on the fact that 100% light return does not necessarily translate to a beautiful diamond.
In this tutorial, I will explain to you what brilliance, fire, and scintillation is and help you determine your light performance preferences.
In diamonds, brilliance is the combination of brightness and contrast.
Brightness is the objective part of brilliance because it is something that can be measured. Contrast, which refers to the difference in brightness levels, can be quite subjective. The contrast pattern you see in a diamond can be created with intense light return, weak light return, light obstruction, and even light leakage. I call the combination of these different types of effects structured contrast.
The point here is that some people will prefer overall brightness, and others will prefer a different kind of structured contrast. In fact, many branded super-ideal diamonds have a structured contrast that doesn’t maximize light return under the table.
Let me explain what I mean with a story from my very first diamond consultation. I was helping my best friend Kenny pick a diamond for his engagement ring. At one point we saw a stone that had a slightly steep crown angle and a slightly steep pavilion angle. On the GIA report, it showed that the crown angle was 35 degrees and the pavilion angle was 41 degrees.
My wife was there at the time so we had her diamond, which was a 34.5CA/40.8PA, to use as a comparison stone. In theory, a 34.5/40.8 CA/PA produces ideal light return.
When we put the diamonds side-by-side, I asked Kenny to tell me which diamond he liked better. He didn’t mind telling me his honest opinion that he actually preferred the diamond with the 35CA/41PA. Under the idealscope, he was surprised that the diamond he chose had very obvious light leakage under the table facet.
What led to his preference?
Kenny explained to me that it was because the arrows pattern stood out more clearly to him. I looked at the diamond and it was obvious that this was indeed the case. The steep/deep diamond was less bright under the table facet and against this weaker light return the arrows were clearly more visible.
You see, in pictures of diamonds where you see black arrows, what you’re actually looking at is a reflection of the black camera lens. At normal viewing distances, the arrows obstruct light and you get darker arrows.
When there is strong light return under the table and the arrows are black, the arrows stand out creating a strong contrast. But when there is no light obstruction, what you will find is that weaker light return under the table creates a stronger contrast with the arrows than in a diamond with strong light return.
The range of angles where obstruction is visible is dependent on the viewing distance and the particular combinations of facets. In general, the closer you view the diamond, the greater the light obstruction. Shallower cut diamonds will have greater light obstruction than diamonds that are cut deep. This is why you need to know what you’re looking at when making comparisons.
In the previous tutorial, I explained how fire is the colored light you see in a diamond. Fire in a diamond is primarily determined by the size and angle of the crown facets. In general, a diamond with a higher crown and a steeper crown facet will produce more fire.
There is a compromise between brightness and fire if you want ideal proportions/light performance and this is known as the cutter’s trade-off.
What I am talking about is the difference between a 34/41 CA/PA, a 34.5/40.8 CA/PA, and a 35/40.6 CA/PA. A diamond with a 34/41 CA/PA is more optimized for brightness and one with a 35/40.6 CA/PA is more optimized for fire. The 34.5/40.8 CA/PA is the most balanced in terms of brightness and fire.
Please bear in mind that all of these CA/PA combinations produce both bright and fiery diamonds and fall within the Tolkowsky Ideal Cut (TIC) range.
Ideally you want to look for a diamond with a 15% crown height to strike a good balance between the fire and spread of the diamond. A higher crown (15.5%) will add some weight and a smaller crown (14.5%) will have less fire. If you have a diamond with a slightly shallow crown angle, it might be a good idea to pair it with a higher crown for a better balance between brilliance and fire.
Brilliant Ideal Cut and Fiery Ideal Cut Diamonds
You can take these basic concepts to the extreme and really optimize a diamond for brilliance or for fire.
A diamond optimized for fire is known as a Fire Ideal Cut (FIC). These diamonds typically have much smaller tables and steeper crown angles with very high crown facets. A diamond optimized for brilliance is known as a Brilliant Ideal Cut (BIC). These diamonds typically have much larger tables, shallower crown angles, and shorter crown facets.
If you’re interested in these types of diamonds, you should note that there are limits to how much you can optimize if you want to remain in ideal light performance territory. The BIC and FIC were cuts coined by Gary Holloway and there has been much debate on what the limits are. If you want more information on these cuts you can read an article by Gary here.
I want you to be aware that choosing a FIC or a BIC really does trade-off a lot of brightness or fire so think carefully before deciding to purchase one of these diamonds. Also, if you’re planning to optimize for fire, you should know that there are other gemstones similar to diamond that actually have more fire than diamond. If you’re interested then you can also check out my reviews on the Asha and the Amora Gem.
Scintillation plays an extremely important part in the character of a diamond because whenever someone says that they want a diamond that “sparkles”, they are referring to scintillation. The sparkle of a diamond is affected by the size, the number of facets, proportions, physical and optical symmetry of the diamond.
Scintillation can be referred to as the play of light or the on-off effect of each facet as it switches from returning varying levels of brightness and color. Sometimes sparkle is referred to as the ‘life’ of a diamond so a diamond that sparkles more is more lively.
Scintillation is a dynamic effect so it can only be observed when there is movement between the diamond, the light source, or the observer. A static image of a diamond displaying fire is not as inviting as the thousands of flashes or ‘events’ that occur when you observe scintillation.
The contrast pattern in a diamond is crucial to how well it sparkles. It is the contrast pattern that determines the size and number of virtual facets in a diamond. A virtual facet is simply a reflection of a real facet and it can even be a reflection of a reflection. For example, an arrow is a virtual facet because it is a reflection of a pavilion facet.
Sparkle is partly subjective because you can have splintery flashes known as pin-flash or you can have bold-flash. You can also choose diamonds that have a mix of pin and bold-flash sparkle. To optimize for pin-flash, you want a large number of virtual facets. To optimize for bold-flash, you want a smaller number of larger virtual facets.
For any given shape and number of real facets, there is a trade-off between the size and number of virtual facets. What you will find is that a hearts and arrows diamond with its perfect optical symmetry maximizes the size and number of the virtual facets in a diamond.
If you increase the number of real facets, you will increase the number of virtual facets and this will increase the pin-type sparkle of these diamonds. In diamonds that are not well cut, increasing the number of facets can also improve the brilliance of the diamond.
There are many branded proprietary cuts like the Solasfera, Eighternity, Leo diamond, and others that have more facets than the standard 57-facet round brilliant. If you love pin-flash, I recommend you go check out some of these diamonds.
Alternatively, if prefer bold-flash then you should look at some antique cut round or cushion diamonds such as the Old European Cut round, the Old Mine Cut cushion, and their modern AGS0 versions. You can also consider going for a step-cut diamond such as an emerald or asscher, which will be less brilliant, but are specifically cut to return large bold flashes of fire.
In small melee diamonds, the virtual facets are generally too small to create significant amounts of fire. If you want more fire out of your small diamonds and don’t mind sacrificing their brilliance, having them ‘eight-cut’ will produce noticeably more fire. Eight cut diamonds have only 18 facets – 8 crown facets, 8 pavilion facets, a table facet and a culet.
Finally, the proportions of a diamond obviously impact its scintillation. The most obvious is the impact that the lower girdle facets have. Short lower girdle facets produce thick arrows and generate bold flashes in a diamond. Longer lower girdles produce thinner arrows so you can expect these diamonds to have more pin flashes.
Since you have to tilt a diamond in order to view its sparkle, the light performance at all tilt angles are actually important. What you will find is that every diamond will begin to leak light when you tilt it enough, and you will be able to see right through the diamond. This is known as ‘windowing’.
The extent of windowing is primarily dependent on refractive index so a CZ with a lower refractive index will display windowing at small tilt angles. Windowing also depends on the proportions of the diamond and deep diamonds will have more windowing. What you don’t want is a diamond that shows too much windowing as this is a form of light leakage and is detrimental to light performance in general.
Light performance has a great affect on the character of the diamond so it’s important to understand your preferences. Whether you prefer a brighter diamond or one that has more fire, a diamond with more pin-flash or one with more bold-flash, there are many ways to optimize for the effect you’re looking for.
I have to say that sparkle must be viewed in real life in order to really figure out what your preference is. This doesn’t mean you have to see a diamond in real life before you buy one, but it is a good idea to go out and look at real diamonds to find out your preferences.
If you just look at videos, it is very easy to conclude that pin flash is better. However, I have found that in real life many people tend to actually prefer bold flash. If you need help figuring out your light performance preferences, please don’t hesitate to get in touch with me and I will be happy to answer any questions you may have regarding a diamond’s brilliance, fire, or sparkle.