Just because a stone is a sapphire or emerald or other gem variety 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 measures used to evaluate the quality of a gemstone have been developed over centuries. Ancient writings from China, Egypt, India and the Roman Empire have described the value factors associated with gems over thousands of years. The valuation of gems observed in the industry today has been an evolving process. One common thread in this process has been the importance of color. As is implied in the term colored stone, the single most important factor to a gemstone is color. Some colored stone experts estimate that the color component of a gemstone can account for 50% to 70% of its value.
Gemologists consider a gem’s body color in terms of hue, tone and saturation. Combinations of these properties help create the colors we observe. I say “help create” because color is not an absolute: it is a perception of the interaction between the object, eye and brain of the observer. In other words, color occurs when seen, and the typical human can see about 10 million different colors! Hue is the base color that first stands out when viewing a gemstone. For example, blue, violet, purple, red, orange, green and yellow are all hues. Combinations of these can form additional hues. For example a common color in tanzanite is purplish blue. The Gemological Institute of America identifies 31 base hues that can be used to describe most gems. That chart consists of the 7 base hues listed above plus 24 combinations of the above.
Tone describes the degree of lightness or darkness of a color. For example, a red stone can be dark red, medium red or light red. The color may also be between two tone values, i.e., medium light red. The GIA tone scale has 11 positions ranging from colorless to black. Black and white are not considered hues. They are the extremes of the tonal scale.
Saturation refers to the degree of intensity of a color. The GIA saturation scale groups hues into cool hues, (including blue and violet) and warm hues (including orange and red). Cool hues can look grayish as their saturation of color decreases, while warm hues can look brown as their saturation level decreases. The GIA saturation scale identifies six degrees of saturation. These range from grayish (in the cool hues) or brownish (in the warm hues) to vivid. Desirable saturation levels for most gems fall in the moderately strong, strong or vivid range.
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. While the most valuable gem will be a 6-vivid on the saturation scale, the most valuable gems will not be the highest tone value of 8. These gems are actually too dark in tone. The optimal tone is typically medium to medium dark, 5 to 6 on the tone scale. 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 or imperfections. Internal characteristics are commonly referred to as inclusions and external characteristics are commonly referred to as blemishes in the gem trade. The types of inclusions encountered in gems include needles (mineral fibers that formed during crystallization), crystals (small mineral crystals of the same or different type as the gem that formed in the host), pinpoints (tiny crystals), feathers (natural separation across an atomic plane), fingerprints (a healed or partially healed break in the gem), color zoning, chips, scratches, etc.
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. Gemologists consider the type, size, number and position of inclusions when considering clarity. They also consider how noticeable the inclusions are. Gemologists study the type of inclusion because certain inclusions pose a greater likelihood that the gem could be damaged under normal wear than others. They study the size because usually the larger the inclusion the more it may detract from the stone’s appearance or pose a concern for damage. The number of inclusions is considered in relationship to the size of the inclusions. Numerous small inclusions typically will not impact the value of a gemstone to the extent that one large inclusion can. The position of an inclusion refers to its location within the stone. For example, an inclusion positioned under the table facet is likely to be more apparent than one located near the girdle.
Minerals form in different environments and from different chemical elements. As a result, some varieties are more prone to containing inclusions than others. The GIA clarity grading standard for colored stones takes this fact of mineralogy into consideration. Colored gemstones are typed as Type I, Type II or Type III. Type I gems grow extremely clean in nature and usually have no eye visible inclusions. Type II gems typically grow with some minor eye visible inclusions. Type III gems almost always form with many eye visible inclusions. Most popular gemstone varieties fall into either type I or type II. With consideration of the type classification, the clarity is graded as Eye-Clean, Slightly Included, Moderately Included, Heavily Included, or Severely Included. It was stated earlier that color is the most important factor in evaluating a colored stone. It should be noted that if a type I gem (such as aquamarine) has numerous, large eye visible inclusions this will have a significant negative impact on value regardless of how strong and pure the color is.
Here, cut refers not to shape of the stone (round, pear etc.), but to how well the proportions, symmetry and polish are executed. 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.
There are many factors to consider when valuing the cut of a gemstone. Brilliance, windowing, extinction, polish and symmetry are all factors that should not be overlooked. Brilliance refers to how bright or glittery a gem is. Windowing refers to the washed out area of body color in a gem. If there is a significant degree of windowing in a gem, one can usually read text placed underneath the stone when the stone is placed in contact with the text. Windowing is common in stones that are cut too shallow. Extinction refers to dark areas in the stone. Extinction is encountered in stones that are cut too deep or have significant pavilion bulge (rounding of the pavilion). Polish and Symmetry refer to how sharply and well defined the relationship is between the crown (portion above the girdle) and the pavilion (portion of the stone below the girdle). When care is taken, all facets will be polished well and the arrangement will align the facets properly for the shape of the gemstone.
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.