The appearance of a colored gem is a combination of many separate factors, each of which is related to, and affected by, the others. It is precisely the complexity of these intertwined relationships that has bedeviled all attempts to quantify quality. And yet, every time a dealer buys a gem, a quick mental analysis is made, usually within seconds. In grading any gem, one must be cognizant of, but not become lost in, the details.
In the following essay, we will examine in detail the features that make up a fine colored gemstone.
The four C’s
Determining the quality of a gemstone involves what jewelers refer to as the “four C’s.” They are as follows:
- Carat Weight
To these, we can also add a fifth C, Color While these factors are well defined for diamond, no universally-accepted system exists for colored gems.
Color: The First C
For a colored stone (any gem other than diamond), color is the most important factor in determining quality.
|Three-dimension view of a color solid. Illustration courtesy of Minolta USA.|
To the color scientist, given an opaque, matt-finished object, there are three dimensions to color:
- Hue position
- Saturation (intensity)
- Tone (lightness or darkness)
For colored gemstones, there is also a fourth factor:
- Color coverage
Hue position. The position of a color on a color wheel, i.e., red, orange, yellow, green, blue
|The relationship between tone (lightness) and saturation.|
Generally speaking, gems with hues that most closely resemble the red, green and blue (RGB) sensors in our eyes are most popular. Thus the colored gem trinity, ruby, emerald and sapphire. But there is much about hue that is a personal preference and will depend upon an individual’s personal taste.
Saturation (intensity). The richness of a color, or the degree to which a color varies from achromaticity (white and black are the two achromatic colors, each totally lacking in hue). When dealing with gems of the same basic hue position (i.e., rubies, which are all basically red in hue), differences in color quality are mainly related to differences in saturation, because humans tend to be more attracted to highly saturate colors. The strong red fluorescence of most rubies (the exception being those from the Thai/Cambodian border region) is an added boost to saturation, supercharging it past other gems that lack the effect.
When judging the quality of a colored gem, tone is an important consideration. Before buying, it’s always a good idea to consider the lighting conditions under which it will be worn. Look for stones that look good even under the low lighting conditions you find in the evening or in a restaurant, for these are typically the conditions under which fine gems are worn and viewed. Also view gems at arm’s length and look for those that are attractive even at a distance. Exceptional gems will look great under all lighting conditions and viewing distances.
Clarity: The Second C
Clarity is judged by reference to inclusions. Magnification can be used to locate inclusions, but with the exception of inclusions which might impact durability, only those visible to the naked eye should influence the final grade. In this way, colored gems are very different from diamond. Indeed, in certain cases (Kashmir sapphires being a classic example), the inclusions can actually enhance beauty and value.
There are two key factors in judging clarity. These are:
Visibility of inclusions
- Size: Smaller inclusions are less distracting, and thus, better.
- Number: Generally, the fewer the inclusions, the better.
- Contrast: Inclusions of low contrast (compared with the gem’s RI and color) are less visible, and thus, better.
- Location: Inclusions in inconspicuous locations (i.e., near the girdle rather than directly under the table facet) affect value less. Similarly, a feather perpendicular to the table is less likely to be seen
Impact on durability
- Type: Unhealed cracks may not only be unsightly, but also lower a gem’s resistance to damage. They are thus less desirable than a well-healed fracture. As already mentioned, tiny quantities of exsolved silk may actually improve a gem’s appearance, and thus, value.
- Location: A crack near the culet or corner would obviously increase the chances of breakage more than one well into the gem. Similarly, an open fracture on the crown is more likely to chip than one on the pavilion. Inclusions in certain positions may also reflect, making a single inclusion visible throughout a gem.