Here are the factors that we consider in determining gemstone value.
Gemstone Species and Variety
The gemstone species is a scientific classification based on the chemical composition of the stone. A given species of stone may have several varieties. The different varieties share the same general chemical formula with only minor differences due to impurities within the crystal structure. But these minor differences can make a huge difference in the color of the stone. Some varieties of stone are common while others are rare (and more expensive).
The example in Table 1 below is for the mineral beryl [ Be3Al2(SiO3)6]. The different varieties of beryl have vastly different values (all other quality factors being equal) based on color, which is caused by impurities in the crystal lattice of the stone. We all know that emerald is much more valuable than aquamarine, but both are essentially the same stone, the only difference being the impurities that were present during their formation. As little as 0.01% impurities in the crystal structure can change the color.
|Variety||Color (Hue)||Impurity causing color||Relative Value *|
|Goshenite||Colorless||This is pure beryl||1|
|Aquamarine||Bluish green to blue||Fe2+ ions replace some Al||3.1 to 6.8|
|Bixbite||Red||Mn3+ replace Al||200 to 500|
|Emerald||Green to bluish green||Cr, Fe, V replace some Al||80 to 320|
|Golden Beryl (Heliodor)||Yellow to yellowish green||Fe3+ ions replace some Al||2.1|
|Morganite||Pink||Mn2+ replace Al||3.5|
Table 1: The relative values of different varieties of the mineral beryl.
The size of the finished gemstone (or a piece of facet rough) is an important factor in determining its value. The relationship between size and value depends on the variety and color of the stone, but is often exponential in scope. Table 2 below shows a hypothetical example of this relationship.
|Size (in carats)||Value per carat||Value of stone|
Table 2: An illustrative example of the relationship between gemstone size and value. This is an example, not taken from any particular stone variety.
As the size of the stone increases the value increases much more than the ratio of the sizes. This is simply because it is more unusual to find larger gemstone material than smaller material. This relationship is highly dependent on the variety of stone and in some cases even dependent upon the actual source of the rough material.
The Color within the given variety
Just as impurities cause the different varieties of gemstones within a species, they also cause variation of color within a variety. The hue of a gemstone is its body-color; the overall color you see. The value (tone) of a gem is its degree of darkness from colorless (white) to black.
The chroma (saturation) is the intensity and purity of color. A low saturation is common in pastel stones, but in darker material may not be desirable. For a full discussion of how we evaluate color, please see the Gemstone Color article.
For many gems a medium to medium dark Value with moderately strong to vivid Chroma are highly preferred. However, many gems with pastel coloration typically have low saturation and can still be quite valuable.
The color of a given gem (hue, value, and chroma) provides its base value. The color of the stone is determined, then a quality number between 1 and 10 is assigned based solely on this color. Gemworld International provides a Color System that contains these quality values based upon color for the type of stone under consideration.
The clarity of a gemstone is defined by the number, size, distribution, and type of inclusions (sometimes called flaws) in the stone. There are many types of inclusions, and rarely they may actually increase the value of the stone. Most often the transparency of the stone is diminished by inclusions, and the reflection of light, called brilliance, is negatively impacted. In some circumstances the durability of the gemstone is affected by its inclusions.
There are many methods for describing gemstone clarity, but probably the best known is that used in the diamond industry, originated by GIA (Gemological Institute of America). RSA Gems uses the following scale to assess clarity of its rough and cut gems.
- IF – Internally Flawless. This means that any inclusions in the stone are invisible to the human eye, even under 10x magnification. For most stone varieties this is an extremely rare designation.
- VVS – Very, very slightly included. Stones with this designation may have one or two very small inclusions that can be seen with a 10x loupe, but cannot be seen with the naked eye. This designation is often called “eye-clean.”
- VS – Very slightly included. One or two very small inclusions that can barely be seen by the naked eye, but can be seen readily under 10x magnification.
- SI1 – Slightly Included 1. Some localized inclusions (not spread throughout the stone) that can be seen by the naked eye. These inclusions are slight enough to not significantly detract from the beauty of the stone, nor do these inclusions pose a hazard to the durability of the stone (for example, no small cracks penetrate the surface of the stone).
- SI2 – Slightly Included 2. One or more obvious inclusions that may negatively impact the beauty of the stone. Very small inclusions may penetrate the surface of the stone or be spread throughout the stone.
- I1, I2, I3 – Included (Light, Moderate, Heavy). This material is best used in cabochons. This material is rarely faceted, unless the inclusions add an interesting, artistic quality to the finished stone.
- Opq – Opaque. These stones are best used for carvings, polished in a tumbler, or made into cabochons.
In order to use Gemworld International’s appraisal system, we describe clarity according to their simpler scale and try to equate the two as in the table below.
|Gemworld International Clarity System||GIA Diamond Grading Clarity System|
|FI (free of inclusions)||IF and VVS|
|LI (lightly included)||VS|
|MI (moderately included)||SI1 and SI2|
|HI (heavily included)||I1 and I2|
|EI (excessively included)||I3|
One significant difference between the GIA and Gemworld International systems is that GIA considers clarity according to the type of stone. They have what they define as Type I, Type II, and Type III gemstones. The type II (often included varieties such as sapphire) and type III (usually included varieties such as emerald) receive less harsh grading than the type I (usually not included such as aquamarine) stones of equivalent clarity. Gemworld International provides no such distinction; all gemstones are evaluated for clarity in an identical manner.
Deductions in quality grade occur under this grading system due to inclusions as follows:
Two more phenomena are also considered under clarity, that of color zoning and of texture.
Color zoning is common in amethyst, sapphire, and some other stones. In some stones there may only be a little color in small zones and clear throughout the remainder of the stone. There is a deduction of ½ point for each of two distinct levels of color zoning.
Texture refers to clarity issues due to tiny microscopic inclusions throughout the stone giving it a fuzzy or milky appearance. Some refer to this as “silk.” There are five levels of texture: Transparent, Faint, Moderate, Strong, and Prominent. Deductions begin with moderate: ½ point, strong: 1 point, and prominent: 1 ½ points.
So if we have a gemstone with the best possible color, giving us a starting quality of 10, but had an excessively included stone with heavy color zoning and prominent texture, the deductions would be 4 ½ points, giving us a Quality of 5.5. While most people would create a cabochon with such poor quality facet rough, it does illustrate how much clarity factors into our overall quality.
There are several factors to assess in the cut of a gemstone. A poorly cut gemstone may be worth as little as half what a superbly cut gemstone of the same material would be. The actual value increase or decrease due to cutting does depend on the variety of stone, but it is a significant factor in the value that is frequently overlooked by the consumer. The following factors are considered in each stone graded by RSA Gems prior to listing them for sale.
- Proportions – This refers to the geometric proportions of the stone, the ideal of which is dependent on the actual pattern that was cut. The depth of the stone’s pavilion must not be too shallow or the light will not reflect inside the stone but instead be lost out the sides of the stone. Too deep a pavilion results in a dark stone, perhaps not as brilliant as it could be, but most of all it may require a custom setting to be used in jewelry.
- Symmetry – Another geometrical comparison, every cut gemstone has a symmetry based on its shape (square, octagon, oval, etc.). Symmetrical facets should be the same size and angles. The sides of the gem should be of appropriate length relative to each other according to the ideal shape.
- Meet Points – This refers to the geometric points where three or more facets should meet. These points should be sharp and distinct as should all facet edges.
- Brilliance – The brilliance of a cut gemstone is defined by the percentage of light that is reflected off the facets within the stone and back out through the stone’s table. While brilliance is affected by inclusions within a stone it is also impacted by the quality of the cutting design, and the execution of the cutting and polishing.
- Polish – Cuts, pits, scratches, and grooves (collectively referred to as blemishes in the industry) are the hallmarks of poor polishing. The actual grit size of the final polish used also has a huge impact on the interior and exterior reflectance of the stone. An excellent polish will not show any blemishes to the naked eye and few to none under 10x magnification.
- Girdle – The girdle of the stone should be even (level) around the stone and of uniform thickness which is between 0.2 and 0.6 millimeters. It is not unusual in commercial stones to see thick girdles in order to increase the finished stone weight. Thick girdles can be difficult for a jeweler to set, and razor thin girdles can cause the stone to crack or chip during setting.
Deductions for cutting are even more stringent than for clarity.
|Brilliance||Up to 1 ½ points|
|Crown Height||Up to ½ point|
|Pavilion Depth||Up to ½ point|
|Girdle Thickness||Up to ½ point|
|Girdle Level||Up to ½ point|
|Table Size||Up to ½ point|
|Symmetry||Up to 1 ½ points|
|Polish||Up to 1 point|
Of course, no one makes every possible mistake in one stone! But the potential for complete disaster on the part of the gemstone cutter should be immediately apparent. By the way, making every mistake would put our total quality below zero…
Most gemstone treatments lower the value of gemstones. Heat treatment may increase or decrease the value of gemstones and depends on the variety. Corundum (Sapphire and Ruby) when heated becomes much less valuable, however without heat treatment most corundum would not be of high enough quality to sell for jewelry. Aquamarine is often heated to obtain a beautiful rich blue color that is more valuable (generally) than its unheated counterpart, which is usually very light pastel in tone. Gemstone treatments, including heat, can alter the value of gemstones dramatically. There are additional factors to consider in heated material in determining value, and these are part of Gemworld International’s system as well.
The most important aspect of gemstone treatments is that they be disclosed to the buyer, and that their price reflects the value when treatments are considered. RSA Gems provides only natural (non-synthetic) gemstones that have not been treated, with the exception of heat. We will always disclose any heat treatment that we know of or suspect and we will not sell any stone that we suspect has been treated in any other manner.
Putting it all together
RSA Gems has investigated many different methods of determining value and we have decided the best integrated system of grading and pricing for wholesale value of a cut gemstone is Gemworld International’s.
At this point, after subtracting all deductions for clarity and cut from the base color value, we have a final quality factor between 1 and 10. These represent quality categories as listed in Table 4.
|1||Very Low Commercial|
|9||Lower Extra Fine|
|10||Upper Extra Fine|
Table 4: Gemworld International Gemstone Quality Categories
Gemworld International has a Gemstone Price Guide (available by subscription here) that is comprehensive for common gemstones. It considers the size and quality category of a stone to determine a value per carat for that particular variety. The value provided is considered a wholesale value for the stone. Gemworld International also provides hands-on training in gemstone evaluation using their color system for grading.
There are other sources of pricing information, and RSA Gems collects information on price from a variety of other sources as well in determining the value of gemstones we intend to sell.
You are encouraged to learn more on gemstone valuation, particularly if you are a collector/investor or jeweler.