The Four C's of Evaluating Colored Gemstones
COLORED GEMSTONE VALUATION INTRODUCTION - THE FOUR "C's"
Grading colored gemstones can be a very subjective process; more so than when grading diamonds. The main factors affecting gemstone price are color, clarity, cut, and carat (weight). They are interrelated, at the same time each factor presents a broad range in itself and will be evaluated accordingly.
I. Color can account for at least 50% of a gemstone's value. Generally speaking, a stone should be neither too light, nor too dark. It should be vivid, rich. Color is evaluated by the GIA (Gemological Institute of America) based on hue, tone, and saturation. These three components are used together to describe an individual gemstone’s color and they will vary.
II. Clarity tends to be the second most important factor in evaluating gemstones. Gemstones are categorized into "clarity types" based on their formation process. Gemstones formed under certain conditions will tend to have more inclusions than gemstones formed under other conditions. There are three clarity types; "Type 1", "Type 2" and "Type 3" gemstones. Clarity will vary in gemstones and within the different types, and affect their value differently depending on which category (Type 1, 2, or 3) the gemstone falls under. Important gemstones such as emerald (Type 3), are formed under less favorable conditions and rarely tend to be "clean", and clarity will be evaluated based on that factor. Gemstones such as Aquamarine (Type 1) have been formed under much more favorable conditions than emeralds and are generally clean stones, so they will be evaluated accordingly.
III. Cut should also be taken into account since it will affect the gemstone's brilliance and sometimes the color hence the gemstone's value. Most gemstones are faceted, in which faces are cut on the gemstone to give the gemstone its brilliance and final shape. A properly cut gemstone will reflect most of the light coming through the stone back to the viewer in an even pattern, giving a stone its brilliance. Some gemstones are not cut at the proper angle (refractive index) at the bottom of the stone, which can cause a "window" in the gemstone (where the viewer can see directly through the gemstone to the other side). Also, gemcutters need to be mindful of how a gemstone is cut and its impact on color. Gemstones become lighter as their size is reduced, which may be a good thing for darker stones, but it may make a light stone lose even more color.
IV. Finally the weight of the gemstone should also be taken into consideration. Gemstones are measured by carat weight (1ct = 1/5 gram). Price per carat will vary among the gemstone groups, and within that based on their specific stone's color, clarity, as well by its size. Gemstones in larger sizes are more rare and thus receive a higher price/carat. Price tends to increase exponentially going into the larger gemstone weights. In the smaller sizes, prices among different gemstones has much less variation.
In order to assess the value of any gemstone, a good understanding of these factors and their variations is needed. Furthemore, these components will assume different values depending of the gemstone in question. As an example weight and clarity are not as important factors for a citrine which is usually free of inclusions and largely available in any size. On the other hand emeralds or rubies which are very rare in large sizes and usually included, the weight and clarity of the gemstone can become as important as the color.