By Loren Brown
Gemstone color is mainly determined by the elements that make up the stone. For example, the mineral species beryl has a complex chemical formula that includes the elements beryllium, aluminum, silicon, and oxygen [ Be3Al2(SiO3)6 ] If trace amounts of the aluminum in the crystal is replaced by chromium or vanadium, the resulting green variety is called emerald. If some iron (Fe+2) ions are present, the resulting bluish variety is called aquamarine. If the stone has other iron (Fe+3) ions the stone is yellow or yellowish-green and is called golden beryl or heliodor. Instead of iron ions if the stone has manganese (Mn+2) ions then the resulting pink colored beryl variety is called morganite. As you can see, very small changes in the chemistry of a rock can certainly make a big difference in its appearance!
Beryl (from left to right: Heliodor, Morganite, Aquamarine, Emerald)
In any individual gemstone mineral species, there is a preferred and more valuable color. This is primarily related to the rarity of one particular color relative to another. For example, emerald is far more valuable than aquamarine even though they are the same gemstone species (beryl).
Further, in a given gemstone variety there is also a preferred color. In the variety emerald, approximately 50% of the retail value is solely determined by the color of the stone. Emerald is classified into at least seven shades of green, each with a different base value. The value of a cut one-carat emerald gemstone may jump from $800 to more than $3000 solely based on the color of the stone. This same dependence of value on color is seen in most gemstone species and their varieties.
Evaluation of Gemstone Color
Anyone grading a stone may have a bias on color depending on whether they are buying or selling the stone. Even when not considering value, no two people are apt to see the color in exactly the same way. To further complicate this, stones may have areas of darker or lighter coloration, inclusions that change the light, and even multiple colors in the same stone.
It is in everyone’s best interest to objectively evaluate the color of a given stone, however, in the end, it is the buyer and seller agreement on price that determines the value.
There is no single internationally recognized authority for colored gemstone color-grading as there is for diamonds. Nevertheless, there is one industry-recognized color-grading system that we use at RSA Gems to color-grade our stones, both rough and cut.
World of Color Grading System
Gemworld International has created a gemstone color-grading system based on the Munsell Color System that has been in use throughout the world for many years. Their system is also tied to current industry data on value. The method is independent of the gemstone type – it works for all gemstones.
Additionally, the Gemological Institute of America has a color-grading system and is similar in many respects to the World of Color system and is equally effective. We try to report the color of our gemstones according to both systems, but our valuation is dependent on Gemworld International’s system.
The Munsell color system divides the visible spectrum into 100 uniform hue sectors. There are 10 main hues, each split into ten equally spaced sub-hues. It becomes difficult for the human eye to differentiate between two colors in adjacent sectors, so the Gemworld system divides each of the 10 main hues into four uniform hue sectors for a total of 40. They are labeled with the two letters of the main hue and then spaced in equal sections 2.5 units apart.
The ten main Munsell hues and their notation is as follows:
YR Yellow Red
GY Green Yellow
BG Blue Green
PB Purple Blue
RP Red Purple
Each hue is followed by a number, 2.5, 5, 7.5, and 10, corresponding to the intermediate values. For example, R 2.5 comes right after RP 10 (think of this as a circle, so the end of the bottom is next to the beginning of the top). 10 B comes right before PB 2.5.
Munsell Color Wheel, © 2014 Gemworld International, Inc. Used with permission.
Value is the lightness or darkness (grey scale) of the color and ranges from 2 (nearly black) to 9 (nearly white). This is what the GIA system calls Tone.
Chroma represents the purity of the color. This is what GIA calls saturation. The Chroma is given in steps of 2. A highly saturated stone may have a Chroma value of 10 or higher, while a value of 2 or 4 adds a grayish component to the color (less pure) and is common in pastel stones. In stones with a low Value (dark), a low Chroma can result in a nearly black, low dollar-value stone.
The full color description is given by the Hue, followed by Value/Chroma. The figure below gives a few examples.
10B page from World of Color system. © 2014 Gemworld International, Inc. Used with permission.
Of course, it is much easier to follow this notation if you have the World of Color book. I would recommend this book to investors, collectors, and jewelers who may have regular occasion to use it.
Now you should understand why accurate color description is necessary. The specific hue in addition to the Value (tone) and Chroma (saturation) can have a strong influence on the dollar-value of the gemstone. In fact, color accounts for 50-60% of a gemstone’s value. Even a poorly-cut stone can have tremendous value, as evidenced each day in the world’s commercial gemstone market.
For more information on value dependence on color, see the Valuation article. There is also a discussion of color and value in the articles on specific gemstones.