Hard Core Popular Science Article: Revealing the Mysteries of Gem Cutting

1. WB precision cutting gemstone

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In recent years, the acceptance of colored gemstones in the domestic jewelry market has been increasing year by year. In addition to top colored gemstones such as ruby, emerald, and Palaiba tourmaline, spinel, rupee tourmaline, and others have gradually entered the public eye and gained more recognition.
Figure 1: Colored gemstones with different cutting techniques.

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Learning the quality evaluation system of colored gemstones is of utmost importance in order to understand them. The main factors that affect the value of colored gemstones include color, origin, size, clarity, cutting, and so on.
This article will focus on the impact of cutting on the quality of colored gemstones, analyzing the considerations of gemstone craftsmen before designing and grinding colored gemstones from the aspects of darkness, brilliance, and brightness, and emphasizing the Helena cut.
01 Cutting considerations based on gemstone raw materials
Gem craftsmen usually consider factors such as darkness, brightness, and brightness when designing gem cutting. Therefore, one principle of cutting and designing gemstones is to reflect as much light as possible back onto the countertop, thereby increasing the brightness and shine of the gemstone and controlling the area of the dark region. However, due to the limitations of the original stone form, due to considerations such as preservation, the cutting of colored gemstones may not always achieve a perfect state.

(1) Darkness
When observing with the gemstone countertop facing upwards, we often observe some dark areas. The presence of these areas may be caused by the absorption, leakage, extinction of light by gemstones, or reflection between observers and objects.
a. The absorption of light by gemstones
Some gemstone materials themselves are very dark, such as blue sapphires with extremely high saturation, where light cannot penetrate and is almost completely absorbed, resulting in a dark visual effect. We often evaluate such gemstones as "too black" (Figure 2).

Figure 2: Sapphire with extremely high saturation; All rights reserved © WB precision cutting gemstone
b. Bottom leakage
If the pavilion of the gemstone is too shallow, the light passing through the gemstone directly shoots out from the facet of the pavilion without being reflected back to the crown, resulting in light leakage (as shown in Figure 3). Simply put, it is as if the observer's line of sight directly penetrates the gemstone. If the gemstone table is fixed on a white paper with words written on it, the observer can observe the words on the white paper through the gemstone table. Therefore, when observing gemstones along the countertop, we can observe gray or darker areas, also known as "window opening".
Therefore, the bottom leakage of the gemstone means that any item placed below the gemstone pavilion will affect the color of the gemstone. When setting loose gemstones into pendants, especially for light colored gemstones, the color of the gemstones can be influenced by the wearer's skin color or clothing. On the contrary, gemstones with high saturation colors that only have slight bottom leakage can have a smaller "window" that can reduce the visual effect of high color saturation, thereby enhancing the beauty of the gemstone.
The propagation path of light in gemstones with different depths varies
The influence of light propagation at different corners of gemstones

c. Extinction
If the pavilion of a gemstone is too deep, the light entering the gemstone undergoes multiple refractions and shoots out from the pavilion at the other end, which also cannot return to the observer's eye, causing the "black background effect" (Figure 3, right). The steeper the profile of the pavilion, the more obvious the extinction phenomenon. Experienced gemstone craftsmen cleverly use "extinction" to improve the visual effect of gemstone colors. For example, when cutting and grinding light colored gemstones (such as spodumene, topaz, and morgan stone), increasing the depth of the pavilion can make the gemstone absorb the light passing through or increase the propagation path of light inside the gemstone, thereby making the color contrast of the gemstone more obvious and increasing the saturation of the color visually. This is the illusion of using extinction to increase the saturation of a gemstone for the observer.
d. Reflection between observer and object
When observing gemstones, the observer usually places them 8 to 20 inches away from the eyes. At this point, the light passes through the observer's shoulder and enters the gemstone, then reflects back into the observer's eyes from within the gemstone. Compared to the light source around the observer, the area where the observer is located is considered to be a dark area formed by light reflection, which is also an important reason for the generation of dark regions in most gemstones.
If the gemstone only reflects white light, there will be no appearance of any dark regions. If the observer wears red clothing and observes a gemstone in an environment that only scatters white light, the gemstone will reflect red. Moreover, the color of other items around the gemstone can affect the observation of the color of the gemstone (Figure 5). This is also an important reason why when grading the color of colored gemstones, it is required to be done in a black, white, and gray tone environment, and observers should wear white clothes.
Figure 5 Palaiba tourmaline (left) and ordinary green tourmaline (right). A small amount of green reflection can be seen on the green blue Palaiba tourmaline on the left, which is caused by the reflection of the green tourmaline on the right. All rights reserved © WB precision cutting gemstone

(2) Brilliance
The application of the term 'brilliance' in colored gemstones is not common, as there is a lack of strict definition, and different people may express different meanings when using brilliance.
Brilliance usually refers to the degree of brilliance of a gemstone, which refers to the colored light reflected from the interior of the gemstone, commonly known as "anti fire". For colored gemstones, their importance is self-evident. In a finely cut colored gemstone, most of the returned light is colored light with a certain saturation, increasing the flickering sensation of the gemstone.
Figure 6 Spinel with high scintillation; All rights reserved © WB precision cutting gemstone
(3) Brightness
GIA used the term 'brightness' in the diamond cutting grading system released in 2004. GIA defines "brightness" as the degree of internal and external reflection of "white" light seen in polished diamonds facing upwards
For colored gemstones, when we rotate them, most colored gemstones should be able to return enough light to produce good brightness, which is also an important principle in cutting and designing gemstones.
Figure 7: A high brightness tourmaline; All rights reserved © WB precision cutting gemstone

02 Gem Craftsman's Choice of Cutting and Grinding Methods
Usually, before evaluating the quality of colored gemstones, it is crucial to understand how gemstone craftsmen choose the cutting and grinding method before cutting and grinding. The factors mentioned in the previous section, such as dark region, glossiness, and brightness, are all important considerations.
(1) The application of dark domain (contrast between light and dark)
The presence of dark regions can affect the color and brightness of gemstones, thereby affecting their value. The traditional cutting method usually weakens the contrast between light and dark caused by dark areas, making the appearance color of gemstones more uniform and soft. However, the strong contrast caused by dark regions can enhance the visual effect of color saturation, causing visual impact. In cutting and grinding methods, the clever use of this light dark relationship may enhance the value of colored gemstones.
Some creative gemstone craftsmen may even use contrast between light and dark to break free from the limitations of traditional shapes and create more dynamic shapes.
Figure 8 Impact of contrast on color perception

Figure 8 shows the effect of contrast on color perception. For all graphic representations, green is the same. However, there is a significantly stronger contrast between green and black compared to gray, and the green on the left gives the observer a more vibrant visual effect, with higher saturation and brightness. This indicates that strong contrast between light and dark can produce a more vibrant visual effect.
Morgan stone usually appears in a low saturation pink color. The figure below shows a Morgan stone cut in a heart-shaped shape. Gem craftsmen have effectively utilized the visual effect of contrast to increase the color saturation of the gemstone (Figure 9). Although this cutting and grinding method causes the gemstone to lose some brightness, the light colored gemstone cut in this way is still favored in the jewelry market. On the contrary, for gemstones with higher saturation (such as sapphire), as shown in Figure 10, strong contrast results in a loss of brightness.
Figure 9 shows a heart-shaped cut Morgan stone, with strong contrast enhancing the visual effect of the gemstone's color saturation. All rights reserved © WB precision cutting gemstone
Figure 10 shows a sapphire with high saturation, with strong contrast reducing its brightness. All rights reserved © WB precision cutting gemstone

(2) Control of Window Size
For the same gemstone material, when it comes to the "window" issue, gemstone craftsmen usually need to choose between large diameter, heavy and larger windows, and small diameter, without windows. For certain gemstone materials with sufficiently high color saturation, gem craftsmen can consider reducing the depth of the pavilion appropriately. The high saturation of the gemstone itself can reduce the impact of the "window" on the appearance and value of the gemstone. At the same time, the choice of this cutting and grinding method may also mean that gemstone craftsmen can obtain higher yields from gemstone raw materials.
As shown in Figure 11, there are two aquamarine gemstones, each with a bottom leakage phenomenon. As the leakage window increases, the color in the middle of the gemstone weakens, resulting in a decrease in the overall uniformity of the gemstone color.
Figure 11: The influence of "window" on gemstones; All rights reserved © WB precision cutting gemstone
For different gemstone materials, the refractive index (RI) of gemstones determines the ideal depth for different gemstones to avoid bottom leakage. The higher the RI value, the shallower the depth of the gemstone. Therefore, diamonds with good cutting quality (RI: 2.417) often have a shallower depth than most colored gemstones, while some colored gemstones (such as crystals, tourmalines, etc.) are prone to "windows" due to their low refractive index and the depth of the cut depth often not reaching the ideal depth.
(3) The balance between brilliance and weight
In the cutting of colored gemstones, for safety reasons, the facets near the waist of the pavilion are often too steep (too deep), while the facets near the tip of the pavilion are too flat (too shallow), with only the facets located between the two having a relatively appropriate tilt angle. This cutting method will result in extinction near the waist of the pavilion, leakage near the pavilion tip, and only a certain degree of shine in the middle part.
From this, it can be seen that when cutting colored gemstones, it is necessary to avoid having too shallow or too deep facets on the pavilion, and to make a certain trade-off between brightness and weight.
(4) The impact of jewelry market preferences
The preference of the jewelry market for a certain cutting method can also affect the choices of gemstone craftsmen. For example, if a gemstone craftsman were faced with a very thin and rare gemstone material, how would they choose? Do you want to cut and grind multiple smaller gemstones from the raw material, or do you want to cut and grind a larger gemstone with a leaking bottom? In most cases, the two cutting and grinding methods mentioned above can often achieve similar values. In some markets, gemstone craftsmen are usually more inclined to cut and grind the raw materials into a larger gemstone in pursuit of large gemstones.
Even if it may have a more uniform color and visual effect, jewelers and consumers usually do not favor gemstones with darker pavilions. This preference of people will affect gem craftsmen, making it easier for them to choose the shallower cutting and grinding method of the pavilion.
It should be pointed out that market preferences are always in a dynamic change, and there are certain differences in people's preferences for gemstone cutting in different regions. Even in the same region, people's preferences for gemstones are slightly different.
03 Novelty Cutting: Taking Helena Cut as an Example

Figure 12: Colorful gemstones with novel cutting techniques: Left: Yellowstone; Right: Topaz; (Image source: Gilbertson, 2016)
Designers often try to break free from conventional thinking frameworks and innovate by utilizing the dark domains (i.e. the contrast between light and dark in gemstones), brilliance, and brightness we mentioned earlier (Figure 12). Even if the color is uneven or the color saturation and brightness are poor, designers can use "window opening" to create unique and eye-catching designs.

Figure 13 Montana Sapphire (Image source: www.wbgem. com)
Figure 14 Montana Sapphire (Image source: www.wbgem. com)

When it comes to novel cutting methods, it must be mentioned that the Helena cut is a design that combines modern and classical elements. This unique and charming cutting technique originated from Jeff Hapeman, named after the capital of Montana. The sapphire produced in Montana has a flat and slightly elliptical shape. For most gemstone craftsmen, elliptical cutting usually has a lower profit margin.
However, by striking a good balance between rigorous mathematical modeling and intuition, Hapman attempted to create an interesting elliptical cut suitable for Montana sapphires, using clever and precise arrangement and combination between facets to produce appropriate light and dark contrast, deepening the colors of these typically soft toned gemstones (Figures 13-16).

The left image in Figure 15 shows a 12.61ct Montana blue sapphire, which is cut from a 6.37g heat-treated sapphire raw stone; The picture on the right shows a 7.90ct Montana orange sapphire, weighing 5.2g and appearing light green with a brown core. After heat treatment, it appears beautiful orange with royal blue edges. These two gemstones both have an oval shaped Helena cut. (Image source: Lucas et al., 2017)
The left image in Figure 16 shows a 4.37ct Montana dual color sapphire; The picture on the right shows a 4.57ct Montana blue sapphire that has undergone heat treatment. These two gemstones both have typical Helena cutting. (Image source: Palke, 2019)
Compared to sapphires produced in Asia and Africa, Montana sapphires pose a more severe challenge to Hapman. Montana sapphire's lighter soft colors, polychromaticity, and higher clarity lead to the necessity of high-quality cutting and increase the gem value brought by cutting. In September 2016, Hapman designed and polished the "Montana Queen Sapphire" (Figure 17).
Figure 17, upper left: Montana sapphire raw stone, weighing 31.98 ct (6.40 g), appears light green-blue in color, with a unique shape of blue hexagonal edges and golden yellow centers. It has a high purity, with only a small amount of crystal inclusions and rutile needle like inclusions visible; Upper right: The raw stone has undergone heat treatment, presenting a deeper and uniform color; Bottom left: The cutting and grinding diagram of the original stone; Bottom right: The finished product cut from the original stone weighs 12.62ct and has a yield of 39.5%. Due to its large size, rarity, and aesthetics, this sapphire is known as the "Montana Queen Sapphire". (Image source: Hapeman et al, 2017)
Conclusion
Born unique, the cutting design of each colored gemstone is a wonderful story, simple but not simple. If you also love gemstones like me, then you must not miss them. You can take a look at our website: www.wbgem.com
References
Part of this article has been compiled from:
【1】 Al Gilbertson (2016) Value Factors, Design, and Cut Quality of Colored Gemstones (Non diamond) The GemGuide, Gem Market News, pp. 1-8
【2】 Lucas A., Hsu T. (2017) Magnificent Sapphires from Montana's Rock Creek Mine Gems&Gemology, Vol. 53, No. 1, pp. 128-129