Cutting Pavilion First Over Crown
To me the answer is the Pavilion should be cut first in almost all cases.
Yes, the crown (on some designs) can be cut first successfully, and if you are an experienced faceter and want to cut that way, it can be done.
That being said I personally think cutting the crown first is a poor way to facet a stone. Mainly because there are too many variables that are difficult to control and/or predict. Also many designs will just not work crown first.
When cutting the crown first, the method is more than likely going to be wasteful of material and if care is not taken, create a poor performing finished stone.
Note: One possible exception to this rule is on contest stones some cutters cut the crown first to get the table and star facets dead on, as that is a particular area on a gemstone that is difficult and contest judges inspect closely. Keep in mind that contest stones material wise are generally some thing common like Quartz or man-made and not of great value.
There are several very important reasons that the pavilion should be cut first.
The pavilion is the main reflector of light and does the majority of the work, the light engine. If the pavilion (light reflector) facets are not cut at the proper angles then the entire stone will lose performance. Particularly in lower refractive index materials the angles and sweet spots are very critical and pretty small. There is not much room for error so any changes to the pavilion angles can dramatically alter the designs performance.
If you are faceting natural materials, most of the common materials range from 1.54 to 1.67 in refractive index and fall in what I would call the lower range.
Zircons (1.93 to 1.98)
Of course there are some other materials, but most of the common faceting materials we all cut are listed. Notice that most of the materials in natural stones commonly cut are in the lower ranges. What this means is that the pavilion angles are very important and critical.
What does this have to do with cutting the crown first?
Well basically because the pavilion tier(s) have to be cut at very specific angles in order for the pavilion to reflect light, the faceter is limited in options and how to approach cutting the stone and design. This is not even taking into consideration what may have to be done to accommodate the natural rough you are trying to facet.
Note: I find that cutting the pavilion first is also very helpful when dealing with any issues or flaws that natural rough may have. Once the pavilion angles are cut properly, then I know the shape of the stone and where every thing is going to line up.
For example 41 degrees is a pretty standard angle for pavilion mains in Quartz and is where the best performance generally is in that material. There is very little margin for error, the angles cannot vary more than a degree or so, at least for most practical purposes. If the angles do vary more than a little bit the design performance is usually changed for the worse.
Note: Yes there are some exceptions, but the ability to vary the angle on the pavilion is very marginal and any large change (over a degree) will impact design performance.
From a cutting stand point what this means is that cutting the pavilion first and establishing the correct angle(s) makes the rest of the stone take care of itself. By this I mean that once the pavilion is cut the girdle and crown depth and placements are quite easy to line up. Basically the pavilion sets the rest of the stone.
The crown can be cut first, but it is the hard way to do it. If you cut the crown first you are just guessing at the pavilion and girdle depth needed to finish the stone.
In my example of Quartz, you need to have a 41 degree main pavilion tier for performance. If you cut the crown first you can calculate the depth needed for the pavilion of the design, but you are assuming that everything will work perfectly. How often does that happen?
If the pavilion gives you any problem(s) you do not have any margin for error. You cannot just adjust the pavilion angles and hope for the best. The pavilion angles have to be in a specific range for performance. So if you are a little short in depth when you cut the pavilion, you cannot just “adjust” the angles to make the design work, at least not with out significantly altering the performance of the design.
The solution is to leave extra room when you cut the crown in for the pavilion to be cut. But the problem with leaving extra room for the pavilion of course is that you will more than likely have much more waste. If you are cutting man-made material or something cheap and waste is not an issue. Then of course it does not matter. But if you are cutting valuable stone, well…
I am paraphrasing this statement a little bit… But recently some one made the statement that when “light” hits the crown surface of a stone it (light) bends depending on the refractive index of the material and if the crown angle is not set right the light does not hit the pavilion correctly.
Basically they are wrong on several counts and correct about the angle. The light bends when it enters and travels though the actual gemstone material, not because of the angle of the crown facets.
The crown facets do effect the light some what and can influence the location of where the light hits the pavilion. But this influence is very minimal, particularly in lower refractive indexes.
The shape and number of crown facets themselves do not really make much difference to the light coming into the stone. The crown facets do contribute to making patterns and splitting with in the design. Look at my “Mirage Cuts” that is one of the principals that they are designed around.
Note: Changing the crown facet angle(s) from the original designer’s intent, can and will significantly impact (usually for the worse) the look of the design. In other words if you pick a design to cut because of how it looks cut or in the ray trace, cut the design as published. Changing the angles will usually lower the stone’s performances and appearance.
But what is really important to the design is the angle(s) the crown facets are set at. The crown angle(s) are what really make a difference.
That being said, the crown angles are not nearly as critical and are much more forgiving when changed than pavilion angles are. The crown angles can usually be varied widely and while they may effect the stones look significantly, the crown angles with in a certain range will not effect the performance dramatically. For example with Quartz the standard angles are generally 28 to 42 degrees for the crown, as you can see a wide range will work.
In other words changing the crown angles will change the performance and the look of the stone but there is a lot bigger range of angles to work with on the crown and changes do not effect the design nearly as critically as pavilion changes do. Changing the pavilion angle(s) as little as one (1) degree can and often will completely kill the performance of a design.
What basically this means is that the crown can be adjusted if need be during cutting but the pavilion cannot be adjusted hardly at all. At least with out effecting performance.
So naturally it makes more sense to cut the pavilion first, because it cannot be changed really, and the crown can be adjusted as need be in the later stages of cutting.
OK, another consideration and reason that the pavilion needs to be cut first is that in many designs the pavilion must be cut first in order to establish the shape/outline of the design.
In most designs that are not high symmetry the pavilion must be cut to establish the shape. If you are cutting many types of designs like Barions, rectangles, ovals, hearts, marquise, and many other shapes, the design has to be cut pavilion first to establish the girdle/outline.
How about cutting an OMNI Barion? Cutting this type of design is not really possible, at least not practical unless you cut the pavilion first.
Another point… If you cut the crown first, unless you can cut the crown of the design evenly to center point. How can you make things dependably even and level? Especially on a low symmetry design.
A lot of designs are not made for the crown to be cut center point. Also if you do cut the crown to center point (if the design is made that way), then you will have unnecessary waste because you will have to grind the center point off to make a table sooner or later.
What all this really means is that while the crown of some faceting designs can be cut first by an experienced faceter. Cutting the crown first is not a very practical or effective way to facet most designs. You can do it if you want too, but doing so generates waste and problems that can easily be avoided by cutting the pavilion first.