Proportions and Brightness

There are very few things in this world that shine bright like a diamond, but not all diamonds are cut to deliver maximum light return and sparkle. In fact, most diamonds are simply cut for maximum carat weight, and the brightness and sparkle factor are never taken into consideration, because diamond cutters realize that the majority of people aren’t aware that the proportions of a diamond have a direct effect on the brightness.

And believe me, this is not a new concept, not at all… diamond cutters have been studying the effect of diamond proportions upon light return, which is also known as brightness, since Marcel Tolkowsky unveiled his diamond design back in 1919, and I suspect that his work was based upon somebody else’s work… so let’s just round things off and say that diamond cutters have been studying the effect of diamond proportions on the brightness of a diamond, for the past one hundred years.

The Tolkowsky Ideal Cut Diamond:

Diagram of Tolkowsky Diamond DesignSo after a bunch of calculations, Marcel Tolkowsky determined that the ideal proportions for a round brilliant cut diamond, were as follows:

  • 34.5° Crown Angle
  • 40.75° Pavilion Angle
  • 53% Table Diameter
  • 59.3% Total Depth

The diagram pictured to the left provides a visual representation of how these sections and measurements relate to each other.

Now before you go running off thinking that I’ve given you the secret sauce of the diamond industry, I want to direct your attention to the right side of the graphic above, which reads “0% Girdle” because that my friend, is what we call a design flaw, or to be more accurate,  a technical impossibility… quite simply, you can’t cut a round brilliant cut diamond without a measurable girdle edge.

Brian Gavin Blue Fluorescent Diamond, AGS #104066186034 Tolkowsky Diamond ProportionsThe diagram to the left is from the Diamond Quality Document issued by the American Gem Society Laboratory (AGSL) for this 1.668 carat, J-color, VS-1 clarity, round brilliant ideal cut diamond from the Brian Gavin Blue collection of diamond which exhibit blue fluorescence. Do you see how the girdle edge has scalloped sections which range from thin to medium? Notice how the thicker areas of the girdle edge line up with the facet point junctions? Apparently Marcel Tolkowsky forgot all about this when he calculated the ideal proportions of a diamond… It’s like forgetting that a beautiful woman has a waistline… Whoops! What was good, old Marcel thinking?

Since a measurable girdle edge, is an integral part of the design of a round brilliant cut diamond, we need to expand on Tolkowsky’s Diamond Design, to accommodate for the girdle section… this is why the crown angle of the 1.668 carat, J-color, VS-1 clarity diamond from Brian Gavin referenced above, has a crown angle of 34.7 degrees and a pavilion angle of 40.9 degrees, which then has an effect upon the table diameter and total depth measurements, but which will not have a negative impact of the diamond, as demonstrated by the fact that it received an overall cut grade of AGS Ideal-0 from the AGSL on their Platinum Light Performance grading platform.

The Effect of Blue Fluorescence Upon a Diamond:

Brian Gavin Blue Fluorescent Diamond Review, AGS #104066186034Since this diamond is from the Brian Gavin Blue Collection of Diamonds with Blue Fluorescence, I thought that I should take a moment to talk about the effect of fluorescence upon a diamond… with the understanding that I actually have this diamond sitting on my desk right now, and I’ve been able to take it for a walk and evaluate it under a variety of lighting environments. I snapped this picture with my camera phone, so don’t expect too much from it, it’s only intended to demonstrate that this diamond is keeping me company this morning… and trust me, like most Leo’s it has a sparkling personality! By the way, you get extra points if you figure out the astrological  reference.

The medium blue fluorescence is not visible within this diamond until it is subjected to black light as can be seen from the picture of the blue fluorescence which is provided on the diamond details page.

Now I realize that this is a J-color diamond, but I selected it because I wanted to demonstrate that it is the proportions of a diamond, and not the body color which controls the brightness of a diamond… and this is where people get all sorts of confused with the terminology that we toss around between the sales floor and the backroom of the store where diamonds are graded. So before we go much further, let’s agree on a few diamond grading terms:

  • Brilliance is used to describe how bright our human eyes perceive a diamond to be. However it is not only the amount of light return, but also the perception of intensity created by the internal and external reflections of white light, as judged by looking at the diamond in a face-up position.
  • Dispersion, which is also known as “fire”, is the flashes of colored light that is created by the facets of a diamond. Technically these flashes of light are actually white, but our eyes break them up into colors like red, green and blue.
  • Scintillation is the contrast which is created within a diamond by the light and dark patterns created by the facet pattern; it is also the overall sparkle effect which is created when the diamond is in motion.

Video of Brian Gavin Diamond with Blue Fluorescence, AGS-04066186034“Brightness” refers to how “Brilliant” a diamond is, in terms of the volume of light being returned upwards through the crown facets, towards the person viewing the diamond, and this has nothing to do with “body color” which is actually a measure of the absence of color in a diamond. As you can clearly see from the video of the diamond which is pictured to the left, this 1.668 carat, J-color, VS-1 clarity diamond from Brian Gavin is nice and bright! In this particular instance, I selected a J-color diamond, because I wanted to demonstrate how proportions have a direct effect on the brightness of a diamond regardless of body color.

By the way, I don’t know whether you realize that each color grade actually represents a range of color, so a J-color diamond can have a depth of color which leans more towards I-color, or more towards or L-color, or rest anywhere in between… in this particular instance, I feel that this 1.668 carat, J-color, VS-1 clarity diamond from Brian Gavin leans more towards the I-color end of the spectrum. Which is not to say that it is an I-color diamond, it is clearly a J-color diamond, but I feel that it is closer to an I-color than it is an L-color, and I don’t feel that it rests in the middle of the range for J-color, because when I compared it to an I-color, J-color, and L-color diamond, under the controlled light provided by a GIA Diamond Light, in a completely dark room, from a side profile, it seemed to belong closer to the I-color than the J-c0lor and L-color master stones.

And it “faces up” quite white too… which is not to say that it’s bright white like an F-color or G-color diamond, because obviously it’s not F or G color.  Notice  how I wove the word “bright” into that sentence? In this case, I’m referring to the brightness of color, like they do on a laundry soap commercial, and just like they do at the sales counter of a jewelry store… No wonder you guys are confused! 😉

And we haven’t even discussed Optical Symmetry yet… Which refers to the precision of facet structure, shape, and alignment, which has a direct effect upon the sparkle factor of a diamond. Shall we keep it simple? Drop me a note and I’ll help you select a stone that is optimized to shine bright like a diamond, and then you can focus on whatever it is that you do…


Leave a Comment: