What does cut mean
Diamond cut has two meanings; the first is the general shape, such as round cut, emerald cut, marquise cut, etc. The second, about which we are talking today, is how well the cut has been executed. There is a cut grading scale used on many certificates, which normally goes Excellent/Very Good/Good/Fair/Poor; however, as we will see later this does not paint a complete picture. There are many shades of grey to diamond cut, and as much art as science.
When light goes into a diamond, one of three things happens. Either (a) it gets reflected as white light, (b) it passes though the diamond and out of the bottom, or (c) it is dispersed into a rainbow. The best diamonds in the world possess a pleasing mixture of these three factors. Too much of one or another leads to a decrease in the other factors, and an imbalance in the appeal of the diamond. It is possible to have a diamond with very good light return that has a modest cut grade, and equally to have one with high polish and symmetry grades but poor return. Harmony and balance are of incredible importance.
In the diagram above, we can see the different facets on a diamond, from the table facet at the top, the star, kite and upper girdle facets which compete the top half, and the lower girdle and pavilion main facets on the bottom half. Each is at a different angle to the other, creating a huge amount of reflective surfaces and angles. The overlap of top and bottom halves of the diamond means that each facet in the top half might be split by up to three facets on the bottom half (these are known as “virtual facets”.
The Three Factors of Diamond Cut
The first factor, reflecting white light, is obviously important, as a diamond that does not reflect light is clearly going to be no good! There is a tendency in the diamond world to assume that light return is the single most important factor in judging a diamond, however it is only useful in context. For example, in the image below, all three images have equal average brightness, but the square on the left is clearly brighter, for having strong contrast between the black and white areas.
This leads to the second factor that must be considered, contrast brilliance (not found on a diamond cert). This is where some light exits the stone through the bottom; it might appear that this is a bad thing, however it depends on the manner in which it happens. The images below should explain the importance of this:
The effect of neighbouring light and dark regions leads to contrast brightness. This is the manner in which the human eyes perceives a region as brighter when it sits alongside a dark region. In the images above, the stone on the left has more white (light reflecting) facets, and the one on the right has more dark areas. However, the diamond in the image on the right appears brighter and more interesting, because the dark areas work to make the white areas seem whiter. Our eyes perceive this as sparkle.
Contrast information gives us more information than just judging the total light return in isolation.
Diamond cutters work to create a harmonious balance of light and dark regions, as this makes the diamond face up brighter, even though it might make the diamond seem less good on paper.
This comes about because of how the human eye works. In the image below, we can “see” a white triangle, even though there is no such thing present, because the eye fills in the gaps! A skilful cutter can use this effect to create a diamond that jumps out. As mentioned, this contrast pattern is not something that is graded individually at present, (though a diamond can be downgraded for a lack of contrast which leads to a low level of scintillation) so diamonds that have the same cut grade can appear very different, or even a diamond with a lower cut grade than another can appear more vibrant than the higher graded diamond.
The third factor to consider is the balance between scintillation and fire.
Scintillation is the return of white light and the way that it “dances” across the diamond as the stone, the light, or the viewer’s eye moves. It is worth mentioning that a diamond is almost never static. Whether on a finger, a neck, or an ear, something is constantly in motion. So any grading system that relies solely on the grading of a static diamond will miss the point! The flashes can move faster than we can perceive, leading to a sensation of life and movement. Scintillation is the cause of the “star” effect one sometimes sees in a photograph of a diamond.
Fire is the breaking up of light into a spectrum of colours. In broad terms, fire comes from the star and upper girdle facets, so a smaller table leads to more fire. The trade off for this is slightly less scintillation. The diagram below traces the path of a single light ray through the diamond, and shows how light is broken up. Fire is either “Fast” or “Slow”. Fast comes from small light sources (such as a single light bulb in a room), slow comes from large light sources (such as daylight).
When you consider that there are normally several light sources above or near you, in addition to diffuse sunlight, you can image how there are an incredible array of light rays entering the diamond from every angle, creating a kaleidoscope of colour with large amount of flashes, such as the image below. Then, when you add in movement, the fire and scintillation move around the diamond in a dazzling manner.
Cushion Cut versus Modern Brilliant Cut, a Comparison
There seems to be an “either/or” school of thought when it comes to vintage and modern diamond cuts, and no end of people saying that one should only go for one particular cut or never go below a certain cut grade. In general, older cuts tended to have more fire, and modern cuts will have more scintillation. Of course, there are exceptions, and also there is a middle ground of cuts, such as the candlelight cut from the 1950s, which had a lovely mixture of both. The main dividing line between the two eras of cutting is the invention of the cutting saw around 1900 which allowed for shape the diamond in new and precise ways, and the scientific paper written by Marcel Tolkowsky in 1919, which heralded the introduction of the modern brilliant cut. This was the last main change to the template for diamond cutting – a diamond cut circa 1925 (for example) will be the almost the same as one cut in 2017. The introduction of laser cutting in the 1970s served to increase the yield for the cutter (bigger diamonds from the same piece of rough) and also to speed up the cleaving of the rough, but it does not change the cut quality or proportions.
Below is an informal comparison of cut and light return from a modern round brilliant and a cushion cut diamond. The modern diamond, on the left, has Very Good grades for both proportion and symmetry, and dates from the last twenty years. The cushion cut diamond on the right dates back to the mid-19th century. The wider facets on the bottom half of the cushion cut diamond serves to break up light more on each individual facet, whereas the round cut diamond has individual flashes of colour. The contrast pattern is more evenly balanced across the cushion cut diamond, making it seem brighter. In the stone on the left, each individual facet seems smaller, so the colours are limited to “one per facet”, and we get more fast fire; in the antique diamond, you can sometimes see a few colours per facet, and more slow fire.
There is no one correct answer to questions concerning cut and cut grades. To state, for example, that one should only consider diamonds above a certain cut grade is to miss the mark, as it removes the human element from the equation. Current research, such as Vision Impacts Sparkle, published in Rapaport Magazine (August 2014) is exploring behind how we “see” sparkle and trying to learn more. The research is based on the stereoscopic effect; this where each eye sees the diamond from a slightly different angle, and how the brain mixes the two images to create one image.
If we consider the image above, the left eye sees the blue image and the right eye sees the same red image, the same object but from a slightly different angle. The brain “merges” the two images, to give the figure on the right. The two images almost (but not quite) overlap. This effect cannot be seen except with a diamond in front of you, and is the biggest difficulty with relying strongly on a cut grade. To go back to an earlier image, lets take a second glance at the fire image:
If we consider this to be the image that the left eye sees, then the right eye will see a totally different fire pattern, because of the slightly different angle. The brain merges the two through something called the “information maximisation principle”, adding together the fire patterns, to create an overall image. This is where subjective decisions as to preference are necessary, as one person’s preferred level of fire or scintillation might be very different to the next person’s, and the levels cannot be judged fully from a certificate or grading. The questions being asked in research labs at the moment include whether one large flash of light is better than two smaller flashes, or whether the colour of the diamond interacts with the fire in the stone. The flash effect is interesting because differences in flash size, location brightness and colour must be interpreted and accounted for.
Gems and Gemology published an article by Al Gilberston (manager of cut research for the GIA) in Summer 2013 concerning “contrast patterns for optimising brilliance in faceted gemstones” which explored this, and concluded that brightness and attractiveness depends not only on light return but also on contrast pattern; it also concluded that the size of the gemstone matters, which might suggest that different proportions might be needed depending on the size of the diamond. This is because the brilliant cut has the same number of facets for all sizes of diamonds, so the flash effect and the fire viewed can change greatly as the diamond gets larger. It was only a relatively short time ago (in Autumn 2004) that they published the foundation article for the cut grade system, and research never stops!
None of this would suggest that the cut grade system is inherently flawed; a diamond that grades Good/Very Good/Excellent will certainly be bright and sparkly. However it would lead me to believe that the full picture is waiting to be filled in. What is clear through all studies, however, that some diamonds are brighter than others to a majority of observers. The cut grade system is a useful tool to explain why (and works well in a majority of cases), but is not the final word on the matter. The essential factors could be summarised as follows:
- Brightness: a diamond should appear bright in a variety of lighting environments
- Fire: a diamond should exhibit a large amount of fire
- Contrast: a diamond should have strong contrast, but not lose too much light through leakage
- Balance: a diamond should appear even and harmonious, and be beautiful all around
These must be mixed together, with an allowance for personal preference and subjective observation ( something which is written loud and clear in every peer-reviewed article on cut I have ever seen!) The most fitting analogy I have ever come across is that while a photograph might show you in great detail what something looks like and be technically accurate, a good painting of the same object will make you feel something deep in your core. A beautifully cut diamond will evoke an emotional response first, which will depend greatly on the person looking at it.
For more reading, have a glance at these articles (the GIA articles can be found online):
Modelling the Appearance of the Round Brilliant Cut Diamond: An Analysis of Brilliance, Gems and Gemology GIA Fall 1998
A Foundation for Grading the Overall Cut Quality of Round Brilliant Cut Diamonds, Gems and Gemology GIA Autumn 2004
GIA’s Symmetry Grading Boundaries for Round Brilliant Cut Diamonds, Gems and Gemology GIA Winter 2011
Contrast Patterns for Optimizing Brilliance in Colored Gemstone Faceting, Gems and Gemology GIA Summer 2013
Vision Impacts sparkle, Rapaport Magazine August 2014