What Makes Some Gems More Valuable Than Others?
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:- Color
- Clarity
- Cut
- 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.
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| 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
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| 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.
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| 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.
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| 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 |
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.
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