UNDERSTANDING A GEMS VALUE

There are various factors that determine the value of a colored gemstone, beyond what most people know as the 4 C's: Carat weight(size) Clarity, Color and Cut. Also other factors and sub-factors like origin, crystal quality, brilliance, color intensity and uniformity, cutting style, quality polish and light performance are some of the most important factors involved.

Cut

The word"cut" means more than just the shape of a gem; it also encompasses the elements of "cut quality." Cut quality refers to how well the gem was manufactured or how well various facets were placed. Combined with the proportions, symmetry and polish, a well-cut gem should have a beauty that not only comes from its size color and clarity but from how the facets interact with light. In other words the resulting Light Performance of the gem. All these factors and their interaction with one another will determine the beauty, desirability and value of a gem.

The quality of the rough material limits the gemstone's final appearance therefore, cutters prefer rough that is transparent and without many inclusions. Some gems are rarely eye-clean so some inclusions become acceptable in those materials. Ruby rough with deeply saturated red color and free from even minor inclusions under10X will produce gems of noteworthy face-up color and appearance, even if the fashioning is poor But if the faceting washes out the color or muddies its color by mixing dichroic colors the cutter work has failed. Also, the rough crystal will be worth less than a cut stone even if the stone is poorly cut, because there is a huge amount of risk in that first step, inclusions, color stone breakage, etc.

Cutting the stone removes much of that risk and shows the buyer what the nature of the stone is. Rough material is always a gamble, you can never predict the result 100% until the faceted stone is completed.

Color

In color science, there are three components to color: hue (red, green, blue, etc.) saturation (intensity or richness of the color), and tone (lightness or darkness) There is a fourth color factor for gemstones: uniformity of color or color zoning. The optimal color range (combination of hue, saturation, and tone) for each gem is different, and for many gems, availability in the optimal color can be more difficult to find. Pale colors usually have a relatively low value. However, pale blue-green (or mint) tourmaline from Afghanistan commands high prices, much higher than their more saturated counterparts from the same source. Retailers have learned that there are customers who prefer less saturated stones because they appear to be brighter and have more sparkle. Therefore, less saturated gems might sell more quickly than the more saturated ones.

Gems that are so dark that it is difficult to see through them are more difficult to sell and their cost usually drops significantly. The optimal color can cause values to significantly spike. In most cases (except bi-colored gems) color uniformity or evenness is also part of this value equation. Gems that face up with multiple hues are usually less valuable than gems that show a single pure hue. For example, evenly blue sapphires have more value than those with secondary green hues. For a colored stone, color is the most important factor in determining its value but not necessarily its quality.

For example, two completely loupe clean tourmalines, with the same size clarity and cut can not be said that they are of different quality purse just because of a difference in color. Just as you cannot say that a brown color Ferrari is of lesser quality than an exact same Ferrari of pure red color. But it is true that there will be a difference in value. In this case, the demand for the color will be the factor determining the final value of the gem.

The demand for color evolves and changes with time. Just as the demand for different cuts and shapes has changed with time. Traditionally pure hues like pure deep red with no hint of orange or purple, command the highest prices. Another important issue with color is that color is not static. Color is a dynamic factor. The color of a gem will change according to the environment and with the type of light. It is well known that the color of a gem is completely different under incandescent or fluorescent light.

Most gem people like grading in natural sunlight. This does not mean that you look at the gem when the sun's rays are hitting directly on it. This will usually cause an extremely high reflection that will not allow proper observation of the gem and will probably blind you. The best natural light is indirect sunlight, like the light that comes in through a south-facing window(northern hemisphere) during the morning to early afternoon. Moreover, if it is cloudy or sunny outside the color will also look different. With time, you will understand that if there is even one single cloud in the sky the color will look different.

If the color of the walls of the room you are in is other than perfect white, it will also affect the color. I remember in my early years in New York when attending GIA, I was observing a morganite under the microscope and I could see a deep red tinge to it that would increase and decrease in strength according to the orientation of the gem. I could not figure out what it was. I knew it was not an inclusion of any sort but could tell something was off. It was driving me crazy. I asked my teacher about it and after some time he told me to look at the roof at the back of the room. As I looked I saw a smoke detector with a small faint red LED light that was always on. I could not believe that tiny red light in plain daytime was the cause of the reddish tinge I was observing. It is because of this issue that photographers and gem traders in general have such a hard time representing an accurate color of their gems in video and photography.

Origin

For many colored gems, the country of origin or the mining location, greatly affects the value. This is truer and more extreme for an untreated sapphire of intense blue color from Kashmir which is worth far more than a similar sapphire mined elsewhere. A few of the major grading labs have the equipment and expertise to determine geographic origin. An origin report from one of these labs is required in validating the gem's value when highly regarded locations can significantly increase the stone's price.

Be cautious and read the report from a lab thoroughly. A standard report usually identifies the gem material, but not the geographic origin. The language used in a standard GIA Identification Report for a copper-bearing tourmaline will state, "This copper and manganese bearing tourmaline may be called ‘Paraiba' tourmaline in the trade. The trade term Paraiba comes from the Brazilian locality where this gem was first mined, however, today it may come from several localities. Some people in the trade call these Paraiba tourmalines even though the location is not from Brazil. Others have chosen to use the term "Paraiba-like" for these copper-bearing tourmalines. Only if the gem was sent in for a country of origin report will it indicate the country such as Brazil. In that case, the price will be impacted by the Brazil origin.

Size

The size which includes the weight and face-up diameter of each type of gem material is also directly related to value. As the size increases to that of high demand, the price per carat goes up. Some gems are rarely seen above a couple of carats (red Beryl) while others can weigh thousands of carats (quartz and topaz). Once gems get beyond a size that is common for jewelry, the number of interested buyers decreases significantly, and the relative value per carat diminishes. However, note that exceptionally large gems from those locations with exceptional color (gems that aren't too dark) are so exceptional that they can be very expensive since they are rarely available from those locations except as near-black gems. Subject to various issues with the rough material, the yield, the final carat weight of the gem compared to the initial weight of the rough, can be as high as 80% or as low as a few percent; cutters evaluate the various pros and cons when planning to cut the gem. The yield from the rough material is usually determined by the clarity, shape and color of the rough piece. In gems like Emeralds, where it is very difficult to find clean rough material, the cutting yield is especially low most of the time. Sometimes the best-saturated colors only appear in larger sizes (Kunzite or Aquamarine), so that small ones of saturated color are rare and surprisingly expensive.

Clarity

Gems are cut to sparkle and show off their color in interesting ways. If there are flaws that interrupt that sparkle, the gem is less interesting. Hence there is a value curve related to clarity that is also true for each type of gem. Some gem materials are almost always found with inclusions while others are commonly eye-clean (inclusions cannot be seen without magnification). There are some inclusions that actually help the value of specific gems, microscopic light-scattering inclusions enhance the color uniformity in Kashmir sapphire by deflecting light into areas it would not normally go. The resulting velvety appearance of Kashmir sapphires adds value. Diamantoid Garnet's horse tail inclusions are also highly desirable and help increase its value when present. In general terms, the cleaner the gem the higher the value.

Cutting

Why do jewelers sell poorly cut "gems," those that only sparkle around the outside with a dull area in the middle? When center (or main) pavilion facets are cut too shallow for that gem material, light passes through so that we see what is behind the gem. This is called windowing. If we can see the girdle reflecting under the table, it is called a fisheye. As dirt accumulates around the edge of the mounting, that girdle reflection will be the color of the built-up dirt (often grey or brown).

These cutting styles are rarely attractive. Poor cutting belongs to a bygone era when customers were not educated or picky enough to care about the quality of their gem. A simple colored stone becomes a real GEM with good cutting. Too many jewelers think poorly cut gemstones are good enough. An exceptionally well-cut gem can add more than 20% to a gem's value. In today's market, a number of gem cutters are known as artists. Gems from named artists can have significant additional value due to the artist's popularity. Even local cutters who are not recognized on a national level can get up to 40% added value for their work with some jewelers.

If the gem were uncut. its value would be significantly less. The impact of cutting on a gem's value is proportionate to its rarity. An extremely rare piece of rough sapphire that sells for tens of thousands of dollars can double in price/carat when cut. It has added value due to the cutting. Top-quality precision cutting or designer cutting done by a famous artist adds a much higher percentage to its value. Native cuts often indicate a cruder style, referring more to an almost outdated method of cutting and thus an assumed lack of accuracy.

There are fewer native-cut gems in the market today. Native-cut gems are usually cut with jamb peg machines. Gems cut by jamb-peg are assumed (but not always) to be cruder in style of cutting and the assumptions are that this style of cutting is poorly executed and less accurate. That is not always the case. In fact, the faceting arrangements that evolved through this method enhance and spread out the color very consistently (remember that uniformity of color is very important), making gems cut in this style have somewhat better uniformity and symmetry very highly prized for some of the most expensive gem materials (ruby, sapphire, and alexandrite).

There are cutting firms that excel at carefully placing facets in those old-style arrangements by both the jamb-peg method and using modern faceting machines. “Commercial Cut” includes many native styles, but the cutting quality is better. Especially, the outlines are even and symmetrical. and there is much better facet symmetry. Commercial cut refers only to a general quality of cutting, not to a general quality of the gem material. In cutting centers that used to be known for native-cut gems, a"master" may still preform the gem using the old cutting style standards, but then use modern methods to finish the gem. Obviously, there is a range of quality for both native cut and commercial cut goods, and the border between the two is often unclear. Just as the border between commercial cut and native cut gems is somewhat ambiguous, the border between commercial and designer cuts is also ambiguous.

Precision Cutting

Precision cutting is done using modern index faceting machines. The focus is usually to cut gems in exact angles with perfectly planed measurements. This allows the creation of intricate faceting designs with sharp facet junctions resulting in perfect geometrical symmetry. Stressing perfect meet points between facets and perfect polish, there are usually three main objectives in precision cutting.

One is to get a perfectly executed cutting with perfect symmetry and alignment o facets. Two is to create intricate designs with a high number of facets which require a lot of work concentration, time and a high level of craftsmanship. Three is to produce a gem with the best light performance possible, considering limiting factors like weight loss and color.

Designer cuts gems are gems where the designer creates a unique face-up pattern utilizing unusual and traditional facet arrangements while using modern precision cutting machines. The goal of most designers is to create a crisp appearance with a unique face-up pattern, whether that is bright and sparkly or purposely windowed as part of the unique face-up pattern.

There are many cutters within the US that cut for local jewelers as well as large-scale cutting firms. Those who repeat these designs are perhaps better-called artisans, but the style can be referred to as designer-cut since it is a break from more traditional styles. Fantasy cuts and artistic cutting include both unusual outlines with standard faceting and standard outlines with concave faceting. Most of these designs have a unique arrangement of polished grooves on the pavilion which create a dynamic outline. Polished grooves on the pavilion (or crown) help to create new patterns of light not possible with conventional faceting. Some artists have found ways to use odd rough, such as Glen Lehrer who uses shallow rough for his Torus Cut. This discussion is limited to faceted styles mentioned above and avoids those borderline areas (carved designs as well as "optical dishes" sometimes placed seemingly randomly on the reverse of a gem).

Polish

One of the most underestimated factors of the equation, the difference between a good and poor polish on a gem is undeniable. The polish of a gemstone is the last step in the gemstone-cutting process. It gives the finishing touches to the gem so it can express the real luster and glitter that corresponds to the original material. This is especially true for gemstones with a high refractive index. like diamond spinel corundum Diamantoid and others.

A perfectly polished facet is almost invisible when looked at and under reflected light, it should look like a mirror. This means perfect reflection. The importance of a good polish has been underestimated for years in the trade. I have observed myself, top line gems like Kashmir sapphire auctioned at the most famous houses, with extremely poor polish that is evident in plain sight. Poorer polish means less value and vice versa Once you inspect closely a perfectly polished gem, you acquire a natural intuitive understanding of its importance and consequences.

Darkness and Brightness

Extinction is considered by many in the jewelry trade to be the dark areas seen when looking at the gem face-up. That's not quite enough information. There are four different causes of the dark or black portions of the pattern observed in a gem:

Adsorption

Absorption is when the material is very dark and light just can't get through it. Virtually the entire spectrum of the visible light passing through the gem is absorbed. This type of darkness is bad. Windowing, light passes through the gem and is not bounced back in any direction by the pavilion facets to the observer. The effect is that you are seeing through the gem. This creates gray and dull areas in the middle of the gem, which are negative factors.

Refractive Index

Refractive index is a measurement of how far a particular gem material can bend light. The higher the RI the less deep it needs to be cut to avoid windowing. That's why well-cut diamonds (RI 2.417) can be cut much shallower and do not window even when tilted. Besides not reflecting light back up to the observer, the disadvantage of a window means that whatever is behind the gem affects the color. When a windowed gem is set in a pendant, the skin color or the fabric it rests on affects the apparent color. This is more obvious in lightly colored gems. 

Extinction

When extinction appears, light goes into the gem and makes at least one bounce off a pavilion facet but it is not returned back to the observer through the crown. The observer sees a reflection from that steep facet that is coming from a low angle outside the gem, compared to the adjacent facets that are reflecting from a bright light source (highly illuminated area). The strong contrast causes the low-angle reflection to be so dark that it is near-black or extinguished. You aren't seeing through the gem, even though you should be seeing a reflection from that facet of something outside the gem, all that is seen is darkness. This is true extinction and is the result of steep angles on the pavilion. The steeper the pavilion, the more extreme the extinction. An accomplished cutter can use extinction to their advantage. For instance, cutting a pale material (Kunzite or Citrine) with a deep pavilion not only allows stronger absorption of light that travels a longer path through the gem (this improves some gems only slightly) but also renders the gem's color more distinct. The resulting extinction seems to increase the saturation of the color through contrast. Purposely choosing angles that create extinction gives the illusion of a more saturated gem, and creating angles that produce a higher light return with minimal extinction creates the illusion of a less saturated gem.

Observer and object reflection.

Observers usually hold a gem 8 to 20 inches away from their eyes. Light is typically coming over their shoulders, entering the gem and returned back for them to see. The observer is reflected as dark compared to the light source around them. This should be thought of as observer and object reflection.

Brilliance and Light Performance

For some, brilliance refers to the amount of "lives a gem has.” For others, the not-so-technical term, brilliance, may not refer to the white light reflected from within the gem, but to colored light. Color is still the most important factor in a gem’s value and in a well-cut gem, most of the light that returns needs to be saturated with color. A poorly cut colored gem will have areas of dullness and weakened color and will lack brilliance. Brightness needs to include the color saturation when talking about colored gems. Unfortunately, good brightness depends upon good color depth to get that desired saturation. If we want to cut a gem that is uniform in color, bright white light reflected from within the gem can wash out that color. Richard Hughes, author and one of the world's foremost experts on ruby and sapphire, points out that in cutting colored gems, generally, the pavilion facets closest to the girdle are cut too steep while those at the culet are too shallow; often, only those in between are cut at the proper angle. This results in three distinct zones: extinction near the girdle, windows near the culet and brilliance in between. Gems with 50% brightness are still considered appealing in the jewelry trade. However, gemstones with too many dark areas (from deep proportions) or windowing (from shallow proportions), and less colored brightness, are not as valuable, less brightness means less value. Uneven patterns also start to have a greater impact as cut quality worsens. 

The art of balancing all these factors in the cutting process is nowadays labeled as Light Performance or light performance cutting. It is not only brilliance anymore, it is the combination of all the factors that affect the face-up light return on a gem. brightness/ darkness windowing/extinction, color and saturation, which will produce a highly sought-after gem, hence more valuable.

This article was written by Luis Merry Del Val 2019.
Based on the original article Value Factors, Design and Cut Quality of Colored Gemstones, written by Al Gilbertstone May 2016, Gem Guide. also included references from Optical Design of Gemstones, written by Jose M Sasian, April 2003, Optics and Photonics News.