by Fred Van Sant
General Considerations: First we should get out of the way those factors which do affect OP (optical performance) but are not inherent in the design itself. Mainly they have to do with the matching up of the design with your particular piece of rough and/or the changing of the sets of angles used. A design which may be fine on a large stone may not have good OP on a small one. While scintillation is an important stone feature, it can be carried too far. Points of light can become too small and color perception reduced. There is a certain size-level in the points of returned light at which it becomes difficult to perceive color differences. This will not happen if you select the design to match your stone size. It is something which is under the cutters control.
To reduce the complexity of this subject, then, we will separate it into two parts: 1) those features which are under the cutters control, and 2) those which are not.
After you have ground off the sharp projections and are ready to choose the orientation, the first consideration is the shape your finished stone will take. In the old days when OP was little understood, carat weight was the main consideration, but we are now free from the tyranny of carat weight, and good OP, which translates into a beautiful stone, is more appreciated.
If, however, you are going to cut for maximum weight, you need not read any further; just let the shape of your rough dictate the design and improvise it.
Before looking at designs you will want to orient your rough by deciding where the vertical axis will run through it–where your table and culet areas will be. Your general H/W (height of stone divided by its width) should be about .7 or 70%. If you plan to use low crown angles your H/W can be as low as 60%.
The other factor in choosing a design is color. If your rough is medium to dark in color you are not going to get much dispersion, so you will probably use low crown angles to lighten the color and increase brightness. If your rough is of light color and low refractive index, such as quartz or beryl, you may want a normal or higher crown to enhance its color. If you have a high-refractive material such as clear or pale- tinted CZ you face two trade-offs as regards dispersion. If you go for maximum brightness–lower crown angles–you should expect to sacrifice some of the dispersion of color. Or if you go for a bright stone center on a round design using even-numbered culet indexing, you will not get as much dispersion around the table as you would with odd-numbered culet indexing, although with odd-numbered culet indexing your table will not be as bright. Decisions, Decisions !
What you, the cutter, can control:
So much for general considerations. Now about the things you have control over: Your main control of OP is in design selection to fit your rough, and how you orient your rough. In addition you can change angles on the crown or pavilion, or both, to adapt a design to your 1-11W or increase its brightness or deepen its color. Without getting into design alterations, there isn’t much else you can do, so now let’s get down to the meat of this topic–design features.
Design Features and Optical Performance:
The general ideas which guide the designer in creating the design should also guide the cutter in the selection of a design.
What is it about the SRB (standard round brilliant) that makes it perform so well? 1) the culet facets are at regular intervals around the stone so that there is a balance in reflection throughout a 360° circle. 2) the pavilion angles are not spread out much–they are all close to optimum for returning light back up through the crown. In relation, the pavilion is as shallow as possible, so light is not lost through so much bouncing back and forth that it never gets back out. 3) the area beneath the table is completely covered by culet facets or by the break facets which are close in angle to the culets. 4) being round, there are no corners which funnel light or prevent even disbursement. 5) the crown has a variety of facet sizes–large for the mains, medium size for the breaks, and smaller ones for the stars, making for a variety of flash size. 6) the pavilion and crown angles are designed to maximize brightness and color dispersion.
As a design departs from the round shape, sacrifices and compromises must be made. Many good designs are the result of finding ways to keep as many of the SRB’s features as possible in shapes other than round. The Barion idea is one such; my Squaround idea is another. Let’s take the SRB’s features one at a time and look at them in other shapes.
(1) Balance in culet indexing: Where the indexing gap between culet facets is wider in some places than others, there is the chance of that part of the stone being darker. If the gap is large it is almost certain there will be a dark area. Look at all culet facets that come to a point as if they were for a round stone, and note deviations. For other sets of facets, circular balance of indexing is also desirable. If there is too close bunching of facets as to either their indexing or angles, it is likely that gaps are occurring between the bunches; also the bunched facets are difficult to cut.
In a triangular or square stone, I usually avoid having a facet coming off the corner on the pavilion at an index half-way between two sides; it makes for a deep pavilion. For example, on a square design, 96 gear, instead of a facet coming off the corner at index 12, it’s better to have two facets bridge the corner by coming off that corner at indexes 10 and 14, or 9 and 15.
If the design is rectangular the same principle applies, but the two facets may have higher index numbers. High 11W ratio oblong shapes are the most difficult to design. For designs with LW over about 1.8 the point culet has to be abandoned in favor of a line culet or keel, and even indexing of culet facets becomes impossible.
In the Marquis shape, light tends to get funneled into the two sharp ends, so to avoid having broad flashes in the center of the stone and pinfire on the ends, the number of facets on the ends can be reduced (much depends on the particular design).
(2) Pavilion angle spread: On round stones, the less spread in pavilion angles the better. Deep acorn-shaped pavilions like the Portuguese Cut tend to trap light. Viewers are not usually aware of the pavilion facet arrangement unless the design is such that a special effect is produced, so it is better to think of the pavilion only in terms of how it will enhance the view of the crown–how it serves as a light engine for the stone.
(3) Area beneath the Table: Facets should be as close to the culet angle as possible. Avoid having facets with angles between 45° and 50° intrude into the area directly under the table, especially if they are opposite each other across the stone. They will usually send light out the back of the stone.
(4) Corners which funnel light: Triangular and square designs with sharp corners tend to funnel light either into or away from the corner. Usually the corner will be darker rather than lighter. Sharp corners are about as far away as you can get from the SRB’s corners, which are all the same angle. Making cut-corners or double cut-corners alleviates this problem somewhat.
(5) Variety in facet size: Designs which produce a variety in the size of the flashes of returned light are in general more interesting (to me) than those which produce flashes all the same size. Especially boring is the stone which shows a zillion tiny pin-point flashes. If you’re going to cut a design with a hundred of more facets, be sure to make it a big one. This is a subject which has not received the attention I think it deserves; I know of no study done on it. Variety can be obtained either by varying the size of facets on the pavilion, or on the crown, of both.
(6) Good Angles: In spite of all we have learned about angles, designs are still coming out with goofy angles. Some designers appear to be happy with just about any old angles they happen to use when cutting a stone. You, the cutter, do not have to accept these angles. You can tangent-ratio them, or if they have too large a spread, ignore the design. The angle spread I refer to does not apply to oblong shaped stones where the pavilion break facets are made steep so that the culet facets can all be close to the same angle, but to those designs where there is no compelling need to have a large spread in angles.
The final answers are not in on this subject, and perhaps they never will be, as to some degree a subjective judgment must be made regarding the best balance between various light characteristics, especially brightness and dispersion. I personally do not consider Brightness important in itself–if I want to look at a lot of white light I can shine a flashlight into my eyes or stare at a turned-on light bulb. For colorless stones, brightness is important only to the extent that those light-paths which the computer program uses are the same light-paths which return dispersed rays of color. To the best of my knowledge this has yet to be proven, but it may lie in the near future, as computers are approaching parallel processing capabilities. Currently, for single ray processing, sodium light is assumed–a yellow color approximately mid-range in the color spectrum. For colored stones, the brightness figure is a good guide to use in your decisions about angles.
I think that for colorless stones the quality of dispersion is most important. As color inherent in the stone gradually replaces dispersion, then it becomes the most important quality of the stone. That is, of course, my own subjective judgment. You may have a different idea about what is important in a cut gemstone; in any event it will be the buyer’s dollar that will decide. If you look at the prices of cut stones for sale you will see that they agree with my values. You do not even see for sale single stones returning just white light.