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:
  1. Color
  2. Clarity
  3. Cut
  4. 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.
and violet. Purple is intermediate between red and violet. White and black are totally lacking in hue, and thus achromatic (‘without color’). Brown is not a hue in itself, but covers a range of hues of low saturation (and often high darkness). Classic browns fall in the yellow to orange hues.

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.

Three green gems, showing a variation in hue position. The round center stone is a straight green, while the trillion-cut stone at left is a more yellowish green and the oval stone at right a slightly bluish green. Generally speaking, hue position is of less importance than saturation. Photo: Wimon Manorotkul
 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.

Four blue sapphires showing a variation in saturation and tone. Stone 1 possesses a light tone and low saturation. Stone 2 is close to ideal in both tone and saturation. Stone 3 has greater saturation than Stone 2 in some areas, but its overall tone is too dark and it shows too much extinction. Stone 4 is so dark in tone that its saturation is reduced. Note that inclusions are far more visible in stones of light tone than those of dark tones. Photo: Wimon Manorotkul

Tone. The degree of lightness or darkness of a color, as a function of the amount of light absorbed. White would have 0% darkness and black 100%. At their maximum saturation, some colors are naturally darker than others. For example, a rich violet is darker than even the most highly saturated yellow, while the highest saturations of red and green tend to be of similar darkness. Note that as saturation increases, so too does tone (since more light is being absorbed. However, there reaches a point where increases in tone may result in a decrease in saturation, as a color “blackens.”
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.
Different levels of clarity are visible here in these spessartine garnets from Nigeria. The oval stone at left is eye clean, i.e., with no clarity defects visible to the unaided eye. In the pear-shaped middle stone, obvious clarity defects are visible, while in the trillion-shaped stone at right, they are even more obvious. Photos: Wimon Manorotkul
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.
Note that cabochon-cut gems generally have poorer clarity than faceted gem. This is because inclusions are more visible in faceted stones than in cabs.

Read the rest What makes some gems more valuable than others 2 ?

Post a Comment