By Jim Perkins (with Bruce Harding)
Copyright 2010

I am often asked how to choose good angles for faceting. Some of the answers were published in 19751 and are now available on the Internet at the address provided in the footnote.  One subject is ‘windowing’, which means that light passes directly through the gem without internal reflections; generally when the table is tilted from ‘normal’ (perpendicular) to a line from the viewer’s eye.

This was discussed in the subject article2, which provides charts for common materials. Each of these lists pavilion slopes across the bottom, and the maximum angle of tilt without windowing is listed across the top. Since nothing else matters in the subject of windowing, these data can be listed simply as below:


Note that quartz with 43° pavilion mains can be tilted up to 4.3° without windowing, but that one with 41° pavilion mains can only be tilted 1.2°. The following rules can be observed:

1. As the RI of the material increases, so does the maximum angle of tilt without windowing.
2. As the slope of the pavilion mains increases, so does the maximum angle of tilt.

So it is always preferred to use a material of high RI and high pavilion main slope. Diamond can be tilted 43° with its typical pavilion main slope of 40.75° because of its high RI.

Although tilt limits for windowing are not dependent on crown main angle or table size it is wise to use crown angles from the charts found in ‘Faceting Limits’ which will work in conjunction with the pavilion angles we choose as they work together.

We must be prepared to make some compromises when choosing our main angles; while maximum tilt angle windowing can be achieved using angles in Zone A of ‘Faceting Limits’, we could be limited by the physical restrictions of our rough or by color factors. Also, for material R.I. 1.6-1.7 the preferred angles are in Zone B.

While ‘Faceting Limits’ does help us make better-informed decisions regarding facet angles, it is still up to the cutter to make choices based on his particular piece of rough.

Below we show a corundum with 38° pavilion mains and tilted about 10°.

Figure 1

Figure 1

Figure 1 is a view using ‘Facet Designer’3, with the pyramid environment provided in that software and a black background; the black areas are ‘obstructions’ of light by the viewer’s head. The dark area in the far side of the table: is what you are seeing of the black background THROUGH the pavilion; the faint pattern you see there is only secondary reflections of light from the crown via other facets.

Figure 2

Figure 2

Figure 2 is an illustration from ‘DiamCalc’4; it shows the path of light if it came from the viewer’s eye (above) – note how it passes through the pavilion. Actually, light flows the other way; this light, if coming from below the gem, would be mostly reflected off the pavilion; only a small percentage would pass into the gem and to the viewer’s eye5, so he sees almost nothing from this source.

1 ‘Faceting Limits’, Bruce L. Harding, GIA ‘Gems & Gemology’ magazine, Fall1975, at …
2 see ’Range of Reflections Through Table’, related Figs. 7A&B on p.82, and charts on p.86.
3 ‘Facet Designer’, Anton Vasiliev, circa 2000; instructions by B.L.Harding, 2004, at …
4 ‘DiamCalc’2.3.0, circa 2004, Octonus Co., Russia/Finland
5 Fresnel’s Law: light hitting a gem externally is partially transmitted and partially reflected;
The greater the angle from normal (perpendicular), the less is the percentage transmitted.