Do gem designs with split mains and odd symmetry perform better than other designs?

The short answer to this question is no. Split mains and/or odd symmetry designs aren’t any better than more traditional designs. Often, they’re not even as good. To some extent, choosing split mains and odd symmetry is a matter of taste. However, there are some significant problems with these choices.

By Jeff R. Graham 11 minute read

Different But Not Better

Faceters use most popular designs, like Portuguese and round brilliant cuts, because they work better in common jewelry settings than other designs. Thus, cutting them is a good business decision. In other words, faceters tend to cut the designs that sell best. This is true even among odd symmetry designs. You’ll find 1, 3, and 5-symmetry gems much more frequently than 7, 9, 11, and 13-symmetry gems. The former are just more practical as jewelry stones.

Split mains and odd symmetry are different but not better than traditional designs. They’re also not new. Many people have used and studied these designs for a long time. In truth, with certain materials, they might prove useful. You can do some interesting things with both of them. However, generally speaking, they’re pretty limited. They don’t offer much in terms of performance.

What Can Split Mains and Odd Symmetry Do?

Some people will state that split mains and odd symmetry perform better than more popular designs. For example, some argue that odd symmetry designs have more light return or brilliance and/or split up light rays in a manner than creates more dispersion. This is just wrong.

These designs can and sometimes do create interesting and/or different reflections and light patterns in gemstones. However, this doesn’t mean they have greater brilliance or dispersion. There’s usually very little measurable difference in performance between designs with split mains and standard mains or designs with odd symmetries and even symmetries. They may even perform worse, depending on the material, size, and design of the gemstone.

Split Mains vs Standard Mains

Let’s examine split mains. Think of these as a tool in your cutting arsenal. With certain designs and stones, you could use them to good effect. Of course, in other situations, they would make a poor choice.

Here’s an example of a standard round brilliant tourmaline design (RI 1.62) with standard mains and a light return of 91.6%.

standard round brilliant

Pavilion: 40.5º breaks, 39º mains; Crown: 42 breaks, 36 mains, 24 stars.

Here’s an example of a standard round brilliant tourmaline design (RI 1.62) with split mains and a light return of 91.9%.

round brilliant with split mains

Pavilion: 40.5º breaks, 39º mains; Crown: 42 breaks, 36 mains, 24 stars.

By tweaking the designs a little bit, I can make any of the designs slightly higher or lower than the others in light return. So, the differences are insignificant.

A Gem Design with Split Mains and Odd Symmetry

Here’s an example of a 72 index, 9-symmetry (odd), round brilliant tourmaline design (RI 1.62) with split mains and a light return of 90.1%.

9-symmetry design

Pavilion: 40.5º breaks, 39º mains; Crown: 42 breaks, 36 mains, 24 stars.

You’ll notice there’s virtually no difference between these three designs as far as light ray trace numbers. The split main designs are just a very small fraction higher or lower than the standard mains. The angles of these three examples are identical for comparison.

The odd symmetry round brilliant with split mains is certainly different but not better in terms of performance. If anything, the 9-symmetry round brilliant performs slightly worse than the standard round brilliant. This is due to the lower number of sides. In a round, fewer sides make the sides longer and the corners more likely to leak some light. So, for example, a 7-sided stone would perform slightly less well than a 9-sided stone. A 6-sided stone would perform slightly less well than a 7-sided stone, etc.

Performance Compared

The differences in performance between all of these diagrams are minor, no matter the symmetry or mains. There are just a few percentage point differences between all three designs.

I used GemRay, not GemRayX. I don’t really care for the GemrayX models and find their dispersion and scintillation numbers inaccurate. (For what it’s worth, the GemRayX numbers are almost all identical to those from GemRay for all three designs).

If split mains and/or odd symmetry made any difference at all to performance, not just appearances, the GemRay numbers would show it.

Standard Round Brilliant

  • Average Brightness (cos) 67.5%
  • Average Brightness (ISO) 74.8%
  • Hints helped for 76% of hit rays
  • Dispersion 12.6%
  • Scintillation 24%

Split Mains Round Brilliant

  • Average Brightness (cos) 64.9%
  • Average Brightness (ISO) 71.8%
  • Hints helped for 67.4% of hit rays
  • Dispersion 12.3%
  • Scintillation 32.6%

72 Index 9-Symmetry Round Brilliant with Split Mains

  • Average Brightness (cos) 63.6%
  • Average Brightness (ISO) 74.6%
  • Hints helped for 77.9% of hit rays
  • Dispersion 17%
  • Scintillation 22.1%

What Do the Differences Really Mean?

While there are no real performance differences in ray traces and light return, there are significant differences in light patterns and cuttability between gem designs with split mains and those with standard mains, as well as between odd and even symmetries.

Take a close look at the following ray traces of the three gem designs we’ve examined.

ray trace - round brilliant

Standard round brilliant tourmaline design (RI 1.62) with standard mains and a light return of 91.6%. Pavilion: 40.5º breaks, 39º mains; Crown: 42 breaks, 36 mains, 24 stars.

ray trace - round brilliant with split mains

Standard round brilliant tourmaline design (RI 1.62) with split mains and a light return of 91.9%. Pavilion: 40.5º breaks, 39º mains; Crown: 42 breaks, 36 mains, 24 stars.

ray trace - 9-symmetry

72 index, 9-symmetry (odd), round brilliant tourmaline design (RI 1.62) with split mains and a light return of 90.1%. Pavilion: 40.5º breaks, 39º mains; Crown: 42 breaks, 36 mains, 24 stars.

All three designs have about the same performance. However, you can easily see they have very different light patterns. As I said before, to some extent, choosing one of the split main/odd symmetry designs is a matter of taste.

Split Mains and Odd Symmetry Observations

Here are some of my observations and experiences.

When Do Split Mains Work Best?

Split mains work best on a stone with a lower number of sides, which means a lower number of facets. For example, split mains will work well on a 6-sided stone but not on a standard round brilliant (16 sides). They also work well on a large stone. They especially won’t work well on stones under 10 mm, unless the stone has fewer sides.

Here you have, side by side, the ray traces for the standard round brilliant, the round brilliant with split mains, and the 9-symmetry design. The stones in these ray traces are about 23 mm.

ray trace comparison - split mains and standard mains

Note how small and blurry the split main designs look, especially the split main round brilliant. The 9-symmetry design looks better but has fewer facets than the split main round brilliant. Now, picture these designs cut in a size more suited to everyday jewelry, like 8 to 12 mm.  

You can easily see why faceters don’t use split mains very often.

What Gemstone Color Saturation Works Best with Split Main Designs?

Split mains will look best on lightly saturated rough. Just picture the ray traces above in red garnet. You won’t see much, and the split mains will make the stone look even darker. Now, picture them in aquamarine. You’ll see more of the facets and reflections (if you use a large stone).

Reflection Patterns

These are my personal opinions. Reflection patterns are important in a gem design, but being able to distinguish the patterns is even more important. Unless you use them in a large stone, split mains will just look too blurry and all the same.

A split main is a split main. They don’t present interesting changes or variations in pattern. After the first couple of stones, they’ll bore most observers. Look at the two split main ray traces. Essentially, the tables of both stones look identical. The table is the center of interest for a gem. Other than some size variation, these designs look the same. A design with more pattern and interest will make a much better, more salable stone.

Cutting Issues with Split Mains and Odd Symmetry

The most important thing to consider when deciding whether to cut split mains or an odd symmetry design is cuttability. That means assessing if the work involved in cutting these designs is worth it.

Time and Labor

Look back at the split main diagrams. Notice the number of facets on the pavilion you’ll have to cut?

Cutting split mains will involve a fair amount of time and labor. If you’re just cutting as a hobby or experimenting with synthetic or cheap natural material, then perhaps time and labor won’t be an issue for you. However, if you’re cutting for money and using valuable natural rough, this is a major consideration.

Split mains mean more facets, so consider the size of the stone. Would you want to cut all those extra facets on a 6 mm stone? Probably not. Standard mains would be a better choice. On the other hand, split mains might work well on a 16 mm stone. You might want to break up the pavilion on a large stone with more facets. (Of course, there are other ways to break up a large pavilion. Split mains are just one option. I prefer other methods).

Just remember, long hours and extra facets won’t automatically yield a more salable or better looking stone.

Settings

Odd symmetry designs will present setting issues, especially for larger stones. If you’re a gem faceter, you might think setting the stone isn’t your problem. If you plan on selling your work to jewelers, think again. Jewelers will almost never buy odd symmetry stones. If you plan on selling your stones to consumers who will want to set them in jewelry, just imagine telling them: “I just cut them. The setting is your problem.” Trust me, that won’t work out well for you in the long run.

Why Split Mains Have No Effect on Dispersion

Dispersion is caused by white light breaking down into its component colors as the light ray travels through a stone. Dispersion is a function of the refractive index of a stone and how far a light ray travels through it. The longer the length the light ray travels, the more the white light breaks up into sparkling colors. However, split mains and odd symmetry affect the length light rays travel very little, if at all.

Comparing Dispersion of Standard and Split Main/Odd Symmetry Designs

Let’s look again at the three gem designs we’ve examined, only this time through GemCad. If all three stones have identical angles and depths, all three designs will have the same levels of brilliance and dispersion. 

GemCad - standard round brilliant

Standard round brilliant tourmaline with standard mains.

GemCab - round brilliant with split mains

Standard round brilliant tourmaline with split mains.

GemCad - 9-symmetry

9-symmetry round brilliant tourmaline with split mains.

Notice that the light rays enter the designs in the same place and travel virtually the same length in all three designs. The split mains and standard mains have absolutely no effect on the length of travel for the light rays. I tested light rays all over these designs. The dispersion is the same across the three designs no matter where the light enters the stones. The different reflection patterns caused by the split mains have no correlation to any change in dispersion.

There’s a slight difference between the 9-sided and 16-sided round brilliant. However, that difference has nothing to do with odd symmetry. You’d find the same difference if you were comparing an 8-sided to a 16-sided round brilliant. The difference is a function of fewer sides, not odd symmetry.

You might hear arguments that split mains “split up” light rays more or that odd symmetry designs cause light rays to “bounce around” more. That’s just false.

Can You Change the Dispersion You Get from a Gem Design?

There are only two ways to get significantly more dispersion out of a gem design. You can cut the same design but with a material that has a different refractive index, or you can change the depth of the gem design.

If you cut a design with a gem material with a higher refractive index, you’ll get more splitting and bouncing of light. Thus, you’ll get a higher dispersion out of the design. If you change the depth of the design, you’ll make the gem deeper. Thus, you’ll increase the distance light travels through the gem and change the dispersion. However, none of this has to do with split mains or odd symmetries.

Conclusion

Split mains and odd symmetry designs can create some different reflection patterns and might work well on large stones with fewer sides. They could yield some interesting gems. However, they’ll require extra time and labor. If you choose to cut them, that’s your personal decision. They won’t have any significant effect on brilliance or dispersion. There’s nothing new, special, or magical about them.