Split Mains and Odd Symmetry Designs


Do designs with split mains and odd symmetry have more performance than other designs?

The short answer to this question is NO, split mains and/or odd symmetry designs are not any better and often not as good as more traditional designs. As I have said, there is a reason odd symmetry designs (1-sym, 7-sym, 9-sym are not common, 3-sym and 5-sym are quite common) and split mains are not used very much. The simple reason is that there are not many designs using 1-sym, 7-sym, 9-sym or split mains that are any good or better than more traditional designs.

Note: To some extent odd verses even symmetry and split mains is a matter of taste and what you like. But there are some significant problems with them.

I have said this before but it bears repeating. Most popular designs (Portuguese, round brilliant, trillion … what ever) are widely used because they are simply better and more useful. Designs that are better performance and more practical to cut as well as for other reasons (like setting) are more widely used. In other words designs that work best are the ones that get used.

What split mains and odd symmetry are is different, not better. They can be useful in certain designs and types of materials. Obviously odd symmetry and split mains are NOT used commonly and frankly are generally not as good on average as the designs we all cut every day. If odd symmetry and split mains were better than what we are cutting now, we would all be cutting them instead of what we are cutting. Split mains and odd symmetry are not new, they have been used and studied by many people for a long time. The fact of the matter is that there are some interesting things that can be done with both split mains and odd symmetry. But basically both split mains and odd symmetry are pretty limited and just do not offer much in the way of performance and practicality that many other design types and symmetries do.

Designs with odd symmetry and split mains do NOT have more actual performance than any other given design. The people that make statements that odd symmetry and split mains are some how better, need to do some more studying and understanding of what is actually happening. The statement that odd symmetry designs have more light return and/or some how split up the light rays differently causing more dispersion is just wrong. These designs can and do sometimes create interesting and /or different reflections and light patterns. The symmetry of the design does not make any difference, neither does split mains as far as dispersion or performance is concerned. What split mains and odd symmetry “do”, is change the pattern of reflections in the designs.

OK first split mains. Are they better? NO, they are not better just different and depending on variables (size, material, design) are often not as good as standard mains, which is why they are not widely used. Think of split mains as a tool in your cutting arsenal that can be used for certain designs and stones with good effect. Of course the opposite is true, split mains can be a poor choice depending on the situation. Contrary to what some people claim there is very little measurable performance difference between a designs with regular mains, split mains and/or odd symmetry.

Here is an example of a standard round brilliant Tourmaline design (RI 1.62) with standard mains. Light return is 91.6%

Split Mains and Odd Symmetry Designs - Tourmaline design (RI 1.62) with standard mains

Pavilion – 40.5 degrees breaks – 39 degrees mains Crown – 42 breaks – 36 mains – 24 stars

Here is an example of a standard round brilliant Tourmaline design (RI 1.62) with split mains. Light return is 91.9%

Split Mains and Odd Symmetry Designs - Tourmaline design (RI 1.62) with split mains

Pavilion – 40.5 degrees breaks – 39 degrees mains Crown – 42 breaks – 36 mains – 24 stars

Note: By adjusting and 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.

Look and you will notice that there is virtually no difference between these three designs (below is the 3rd design) in the light ray trace numbers. The split main designs are, just a very small fraction above or lower than the standard mains. The angles of these three examples are identical for comparison (as you can see). Think an odd symmetry round brilliant with split mains is better? The answer is NO in performance. Just different is all.

Here is an example of a 72 index design 9-symmetry (odd) round brilliant Tourmaline design (RI 1.62) with split mains. Light return is 90.1%

Pavilion – 40.5 degrees breaks – 39 degrees mains Crown – 42 breaks – 36 mains – 24 stars

If anything the performance of the odd 9-Sym round brilliant is slightly lower than the standard round brilliant. This is caused by the lower number of sides (lower numbers of sides in a round makes the sides longer and the corners more likely to leak some light). For example a 7 sided stone will be slightly less than a 9 sided stone in performance and a 6 sided stone will be less than a 7 sided stone and so on.

The performance differences between all of these diagrams are minor, no matter what the symmetry or mains are. I am using Gemray, not GemrayX. I find that I do not really care for the GemrayX models and find the dispersion and scintillation numbers not accurate. But for what they are worth the numbers are all almost identical in GemrayX (as well as Gemcad) in all three of these designs too. There are just a few percentage differences between all three designs. If split mains and/or odd symmetry made any difference at all and was some how better as some people mistakenly claim the performance differences would show up in Gemray or GemrayX. The fact of the matter is that there is no real differences. Like I said odd symmetry and split mains are just a little different in appearence, there is no performance difference.

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

OK, so what are the differences between split mains and standard main designs? Odd verses even symmetry? Well basically like I have just demonstrated there is no real performance difference in ray traces and light return. However there are significant differences in the light patterns and cutting practicality between split mains and standard mains as well as odd symmetry designs.

RB standard mains

Standard round brilliant Tourmaline design (RI 1.62) with standard mains. Light return is 91.6% – Pavilion – 40.5 degrees breaks – 39 degrees mains Crown – 42 breaks – 36 mains – 24 stars

RB split mains

Standard round brilliant Tourmaline design (RI 1.62) with split mains. Light return is 91.9% – Pavilion – 40.5 degrees breaks – 39 degrees mains Crown – 42 breaks – 36 mains – 24 stars

9-Sym RB split mains

Round Brilliant 72 index design 9-symmetry (odd) Tourmaline design (RI 1.62) with split mains. Light return is 90.1% – Pavilion – 40.5 degrees breaks – 39 degrees mains Crown – 42 breaks – 36 mains – 24 stars

Look at the Ray traces above closely. It is obvious that all three designs have about the same performance. The main difference is light patterns, as you can easily see. Like I said before some of this comes down to what you like, it is a matter of taste to some extent.

Split Mains and Odd Symmetry Observations

Here are some of my observations and experiences.

Split Mains

They work to best effect on either a large stone or a stone with a lower number of sides, which means a lower number of facets. Split mains particularly do not perform well on a stone under 10mm, unless the number of sides is lower. By that I mean split mains work well on a hexagon (6 sided) stone but are a poor idea on a standard round brilliant (16 sides) because of the number of facets as related to the size of the stone. Look at the ray traces and you will see what I mean

From left to right: standard round brilliant – round brilliant with split mains – A 9-sym round brilliant on 72 index The stones in the ray traces above are about 23mm (on a 600 x 800 computer screen).

Note how small and blurred looking the split main designs are, especially the split main round brilliant. The 9-Sym is better, but it has a lot less facets than the split main round brilliant. Now picture these designs cut in a more every day size like 8-12mm (less then half of the size in the ray traces pictured above). You can easily see why split mains are not used very often. At least in natural gemstones.

Saturation

Split mains work best on lightly saturated rough. Why? Well picture the above ray traces in a red Garnet. Right you will not see much and split mains will actually make the stone look darker. OK picture them in a piece of Aquamarine. Yes, you will see a lot more of the facets and reflections. But the stone will need to be large like I have noted.

Patterns

This is my personal opinion. Reflection patterns are important in a design and being able to distinguish pattern(s) is even more important. Split mains as the stone gets smaller tend to just blur and all look the same (that is one reason I say use them in a large stone). Also split mains tend to be all the same. A split main is a split main. There is no interesting change or variation in the pattern, which means after the first couple they become boring to the eye and most observers. Look at the two split main ray traces, they are essentially identical in the table of both stones. The table is center or interest, other than some size variation they are the same and boring. A design with more pattern and interest will make a much better stone.

Probably the most important difference to me is the cuttability of split mains and how the labor/practical/performance differences work out. What do I mean? OK, look at the split main diagram above. Look at the number of facets on the pavilion split mains.

Labor

Split mains are a fair amount more labor and time to cut. Labor may or may not be an issue with you depending on the stone and your goals. But labor does play a significant role in cutting for money and natural rough, so it is a consideration. Remember more labor and extra facets does not mean a more sellable or even better looking stone.

Performance

Split mains are some times worth doing and sometimes not. What I mean is that split mains will enhance the appearance/looks of some stones and will damage the appearance of others. There are virtually no real performance differences in light return, but there are some reflection differences (see above).

Practical

Split mains are more facets, so the size of the stone you are cutting will be an issue. Just ask your self do you want to cut all those extra facets on a 6mm stone? Probably not, so on a 6mm stone you will likely go for the standard mains. On a 16mm stone you may want to use the split mains, because of size and you may want to break up the pavilion more with more facets.

Note: There are much better ways to break up large pavilion than split mains but they are one way to do it. I prefer other methods in general and seldom use split mains.

Odd Symmetry

Of course with odd symmetry there will be setting issues, particularly as the stone size gets larger. Do you think setting is not your problem? Just try telling your significant other “Honey, I just cut them, setting is your problem…”. Trust me that does not work. As for jewelers, forget it they almost will never buy an odd main stone.

So why would split mains and odd symmetry be worth cutting some times? The answer is that there are some reflection differences and on certain sizes and types of stones odd symmetry and split mains can be useful. Beyond the practical aspects it boils down to a matter of taste. Use them if you like them. There is no performance difference in light return or dispersion.

One last thing. Some people seem to think that putting split mains on a pavilion increase the dispersion of a design. This is false, there is no dispersion difference. There is a looks/reflection difference like I have noted above but that is a matter of taste.

Why no dispersion difference? Here is the definition of dispersion – dispersion is caused by white light breaking down into its component parts (colors) as the light ray travels through a stone. Dispersion is a function of the refractive index of a material and how far a light ray travels through a material (stone in our case). Split mains and odd symmetry affect the length of light ray travel very little if at all.

Note: I do not agree with GemrayX and it’s model of dispersion. If you look at all three of my examples the length the light rays travels in all three designs is basically identical. By definition the dispersion will be very close in all three examples. Actually you will see that in GemrayX the dispersion numbers are actually low and pretty close together, so while I really do not agree with GemrayX it is basically saying the same thing I am.

Screen Shot

Left: A Gemcad screen shot of the Tourmaline round brilliant with standard mains.

Like I said dispersion is caused by white light breaking down into its component parts (colors) as the light ray travels through a stone. So by definition if all three stones are identical in angles and depth all three designs will have the same levels of performance and dispersion (or insignificant differences between them). The longer the length of travel the more the white light breaks up into it’s component colors. Now look at the three Gemcad screen shots I have posted here. Notice that the light rays enter the designs in the same place and have virtually the same length of travel in ALL designs.

Screen Shot

Notice that the split mains and standard mains have absolutely no effect on the length of travel of the light ray(s). I have test light rays all over all three designs. They are virtually the same no matter where the light rays enter the designs. As I have noted there are reflection differences (scintillation), but the light rays all enter the crown and exit the crown in virtually identical ways and lengths in all of these designs.

The Split mains do not change a thing as far as dispersion and the light rays still reflect exactly like the standard main design. There is absolutely no “splitting up into multiple rays” or “bouncing” of the light rays caused by the split mains or odd symmetry. Split mains and odd symmetry make no difference, the light rays in the standard round brilliant and the split main round brilliant designs travel exactly the same paths in and out of the stones.

Screen Shot

The split mains and odd symmetry do not change a thing and the light rays still reflect almost the same as the two standard round brilliant designs. There are some slight differences because of the 9 sides instead of 16 sides in a standard round brilliant. But nothing you would not see if the design was 8 sided for example, instead of 9 sided. The slight differences is a function of the number of sides, not odd symmetry. There is absolutely no “splitting up” or “bouncing” of the light rays caused by splitting mains or odd symmetry. The people who are making the statement that split mains and odd symmetry designs bounce the light rays around more or split the light rays up because of odd symmetry and cause more dispersion are wrong and have obviously not done their home work and/or have failed to grasp the concepts of quality designs and dispersion.

The only significant ways to change the dispersion of a design is to change the refractive index of the material the design is cut in and/or change the depth of the stone/design. Changing the depth and making the design deeper of course makes the light rays travel through more material and increases dispersion.

Note: This applies to all lower refractive index materials. If you get into high refractive index materials the rules will change and you will see more splitting and bouncing around of the light rays because of the higher refractive index. But split mains and odd symmetry are not a significant factor.

Split mains and odd symmetry are nothing new there is nothing magic or particularly special about them. A lot of people have been there and done that including me. If you like split mains and/or odd symmetry then use them, they some times make an interesting stone. But they are certainly not better, or more performance than any other designs.

About the author
Jeff R. Graham
The late Jeff Graham was a prolific faceter, creator of many original faceting designs, and the author of several highly-regarded instructional faceting books such as Gram Faceting Designs.
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