The value of a colored stone is dependent on a number of factors. Just because a stone is a sapphire or emerald does not automatically mean it has value. Most gem deposits typically produce much more low quality rough than they do rough with gem potential. Minerals such as diamond, corundum and quartz have industrial applications, so even in low quality form they are desirable to mine. But others do not. What are the factors that determine if a piece of gem rough has value?
The 4C’s are well established for describing diamond quality. But their application to colored stones is more variable. Color, clarity, cut and carat weight (size) are judged within a different context in colored gems than in diamonds. In addition to the 4Cs, factors including country of origin and whether or not the gem is enhanced are all essential to identifying and understanding its value.
Color: Virtually all colored stones are evaluated primarily by their color. So much so, that certain color calls determine the varietal classification of a gem, i.e., ruby versus pink sapphire. Yet these boundaries are often not well defined and can present major issues for the trade.
Color is considered to be a three-dimensional concept and includes the hue (color name), tone (darkness and lightness) and saturation (vividness). The relationship between hue, tone and saturation is important in valuing a colored stone. The body color that we observe in the gem is a combination of these properties. Usually, as a gem becomes more saturated, it will also have a darker tone. Different gems have different optimal tone values based on what nature provides us. An example of this is aquamarine, which almost always has a lighter tone of blue vs. sapphire, which often has a dark tone of blue. Both may be the same hue, but the optimal tone expected to see in the market is much different for both. When examining a gemstone, be aware of the light source used to illuminate the gem. Ask yourself if the stone have the same appearance in normal viewing conditions.
Clarity refers to the relative degree to which a transparent gemstone is free of internal or surface characteristics. Internal characteristics are commonly referred to as inclusions and external characteristics are commonly referred to as blemishes in the gem trade. Clarity is judged within the framework of the stone’s species and variety. This is done to account for differences in the formation environments for the various minerals. Case in point, it is extremely rare to find a naturally eye-clean emerald, whereas an amethyst is expected to be inclusion-free.
In general, clarity judgements are based on the obviousness of the clarity characteristics of the stone to the unaided eye. This evaluation is done based on the size, nature, number and location of these characteristics in the stone by an experienced grader. Inclusions are not necessarily a negative feature to a gem. Certain gems cannot exist without them. Cat’s eye chrysoberyl and star sapphire require needle inclusions to create the desired phenomena.
Carat weight: Size of a gemstone is generally expressed by its weight. The weight of a gem is measured by metric carat, which is 1/5 of a gram. Traditionally gem prices are always given as per carat price. Although there is no magic formula to assess the correlation, as the weight of a gem increases so does the per carat price. Of course this is not consistent. For example, the price per carat of an extra fine ruby will increase far more substantially with size than would be the case for amethyst. This is because as the size of the stone increases, the population of comparable stones decreases. Price premiums for natural (non-treated) material will increase with size and in some cases dramatically. Here again it is important to note that, although this tenet of value exists, its effect is not consistent across the spectrum of size and quality combinations. For example, the expected premium for a 3.00 ct. natural fine ruby or sapphire is greater than would be observed for 1.00 ct. size of comparable quality. Yet that same premium would be insufficient to accurately estimate the price for comparable quality gem weighing 8 carats. If the gem is truly exceptional, then the premium will be quite possibly more. Also note that some gem varieties that can occur in extremely large sizes would not increase in price when it becomes considerably large. An example here is amethyst or blue topaz. It is not hard to find a 100-carat gem or larger and these are not very conducive to jewelry use. The price per carat may decrease dramatically and often these large gems drop in price to just a few dollars per carat.
Cut refers to how well the proportions, symmetry and polish are executed rather than its shape. How well a gem is cut can greatly affect the overall value because of its influence on color. Well-cut stones bring out the beauty and brilliance more, thus increasing the overall grade. Poorly cut stones will see light leak out of the stone instead of being returned to the viewer’s eye.
Cut is the property that ties color, clarity and carat weight together. A skilled cutter brings out the best color in the rough gem and enhances its beauty with brilliance. Since the physical and optical properties vary from one species to another, there is no standard set of cut proportions for colored gems. Each piece of rough is unique. The cutter is ultimately balancing considerations of weight and quality to achieve desired value.
When combined, the evaluation of color, clarity and cut result in a total grade that considers these three factors. While no grading standard exists, our own internationally used publication, the “GemGuide” by Gemworld International, Inc., uses terminology of Commercial, Good, Fine and Extra fine to refine this step.