CliffsNotes are study shortcuts. They’re intended for students who don’t have enough time (sic) to invest in fully reading a text. They deliver a summary synopsis intended to slenderize full information down to essential points.
Last weekend a quarantined self-gifter asked for my guidance to buy a round brilliant diamond online. No amateur, this. She’s assembled quite a collection of sizes, shapes and qualities, but has always purchased in-person. In order to make this online purchase she wanted a no-fail combination of weight, color, clarity and (drum roll, please) cut-quality.
We quickly determined carat-weight, color and clarity according to her long experience and personal preferences. Where she needed help was navigating the undulating waters of brightness, fire, and scintillation variety within the “Excellent” cut grade: As any diamond enthusiast knows, that optical ocean is wide-ranging. It can be shallow or it can be deep (see what I did there?).
Prerequisite reading
If you have not read my article explaining the difference between dispersion (physical) and fire (physiological), click over and scan down the page. It’s extremely useful for describing to diamond enthusiasts “why” they see what they see in one make of round diamond versus another.
Balanced optical qualities
After we discussed taste factors, including her preferences regarding fire, brightness and related components of contrast which drive scintillation character she decided that the safest road, when buying online, would be to chase a balance of those qualities.
Old school numbers
At first my self-gifting friend did not want a bunch of angles and ranges. She asked for a table% and depth% combination, in the old-school style. I told her I’d keep it as simple as possible, but no one or two measurements work independently. Diamond performance is dependent on the sum of the collective measurements and actually far, far more (in terms of nuance) if we want to get into 3D analysis.
With the understanding that there’s far more depth to this topic than we could possibly cover in a phone call, I promised to limit my Cliff’s Notes to five basic measurements.
My requirements
- A maximum of light entering the crown is returned to the viewer (serving critical-angle)
- Light rays entering the diamond travel through enough material for dispersion to propagate (serving refractive-index)
- Geometry is not too deep (windowing creates darkness) nor too shallow (obstruction creates darkness)
By staying in the middle of deep and shallow extremes, with robust critical angles and ray paths which disperse suitably we can ensure a balance of brightness and fire. Before I suggest how to do this, check out these unbalanced “bookend” models which I recently ray-traced.
- The first model does not promote abundant brightness.
- The second model does not promote visible dispersion.
Antique OEC style model
When this single ray of white light enters the diamond, it (1) reflects once at bottom left, (2) travels to the right and heads up so that (3) a portion of the ray exits the diamond. Remember that the farther a ray of light travels within diamond material the more it becomes separated into its component spectral (rainbow) colors thanks to diamond’s R.I. The ray travels a long way in this diamond so the separation of white light is exaggerated, as you can see at (3).
The visual result will be more perceived rainbow colors, or fire, when that dispersed light reaches the pupil of a human eye. However, there is a trade-off. You can see that abundant light at (3) is reflected back into the diamond. In fact around 45% of the light ray ultimately escapes without returning to the viewer. So while very dispersive, this model is quite dark around its compass.
Modern RB with shallow crown
Alternately, when a ray of white light enters the model below it also (1) reflects on the left bottom, (2) travels to the right, then heads up to exit the diamond (3). But it doesn’t travel through nearly as much diamond material. So the degree to which that light ray becomes separated into its spectral rainbow of colors is lessened. You actually need to zoom in on the image to see the simulation’s indication that dispersion is, in fact, happening.
The visual result is that many exiting rays will be too narrow to be perceived by the eyes as colored flashes (unless you blast the diamond with small-direct light sources). However, the rays take such a short journey that they exit with far more intensity and efficiency, so even though your eyes don’t see as many rainbow colors in normal lighting, the appearance of the diamond is very white and bright. It’s worth noting that some light at (3) is reflected back inside the diamond and lost, but it’s a small percentage – similar to ‘glare’ reflection-energy for light entering perpendicular to the table – essentially negligible.
Cut Quality Cliff’s Notes
If a round brilliant diamond is fundamentally sound in terms of transparency, clarity & finish and has suitable cut-consistency around the compass of the diamond: Accommodating these five criteria will ensure great brightness as well as ray paths which travel far enough through the diamond material to promote good dispersion in normal lighting, with no obstruction or windowing.
- Table Percentage: 56-58%
- Crown Angle: 34.5-35.0◦
- Pavilion Angle: 40.6-40.8◦
- Lower-Girdle Percentage: 75-80%
- Girdle thickness: Thin, Med, Slightly-thick
- Depth (no worries – confined to the above the depth will take care of itself)
Full disclosure
Marcel Tolkowsky provided a similar basis 101 years ago, with the exception of the lower girdle percentage. But he made his calculations prior to electric lighting. In today’s modern illumination scenarios those lower girdles play a major role in balancing contrast.
So there’s nothing too new under the sun here. But I thought it would be interesting to share the bookend-science and reasoning. It may be helpful if you have anyone itching to buy a diamond via e-commerce who’d like to chase the same balance of visual qualities as my friend.
