Quality in colored gemstones
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. When all the minutiae has been pored over ad infinitum, take a step back and simply look at the gem. In the age of high-powered microscopes this may constitute a radical concept, but one which is necessary.
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:
To these, we can also add a fifth C, Color
Color: The First C
For a colored stone (any gem other than diamond), color is the most important factor in determining quality.
To the color scientist, given an opaque, matt-finished object, there are three dimensions to color:
For colored gemstones, there is also a fourth factor:
|Left: Three-dimension view of a
color solid. Illustration courtesy of Minolta USA.
Right: Hue position describes the position of a color on a color wheel. Illustration © R.W. Hughes
Hue position. The position of a color on a color wheel, i.e., red, orange, yellow, green, blue 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
Impact on durability
Cut (make’): The Third
The function of the cut is to display the gem’s inherent beauty to the greatest extent possible. Since this involves aesthetic preferences upon which there is little agreement, such as shape and faceting styles, this is the most subjective of all aspects of quality analysis.
|The parts of a faceted gem. Photo: Wimon Manorotkul|
Evaluation of cut involves five major factors (in no particular order):
Shape. This describes the girdle outline of the gem, i.e. round, oval, cushion, emerald, etc. While preferences in this area are largely a personal choice, due to market demand and cutting yields, certain shapes fetch a premium. For most colored stones, ovals and cushions are the norm. Rounds and emerald shapes are more rare, and so receive a premium from about 1020% above the oval price. Pears and marquises are less desirable, and so trade about 1020% less than ovals of the same quality. The shape of a cut gem almost always relates to the original shape of the rough. Thus the prevalence of certain shapes, such as ovals, which allow greatest weight retention.
|Shapes (girdle outlines). Illustration © R.W. Hughes|
Cutting style. The cutting style (facet pattern) is also a rather subjective choice. Again, because of market demand, manufacturing speed and cutting yields, certain styles of cut may fetch premiums. The mixed cut (brilliant crown/step pavilion) is the market standard for ruby and sapphire, while the step (emerald) cut is the standard for emerald.
Proportions. The faceted cut is designed to create maximum brilliance and scintillation in the most symmetrically pleasing manner. Faceted gems feature two parts, crown and pavilion. The crown’s job is to catch light and create scintillation (and dispersion, in the case of diamond), while the pavilion is responsible for both brilliance and scintillation. Generally, when the crown height is too low, the gem lacks sparkle. Shallow pavilions create windows, while overly deep pavilions create extinction. Again, proportions often are dictated by the shape of the rough material. Thus to conserve weight, Sri Lankan material (which typically occurs in spindle-shaped hexagonal bipyramids) is generally cut with overly deep pavilions, while Thai/Cambodian rubies (which occur as thin, tabular crystals) are often far too shallow.
| If a gem is cut too shallow, light will pass
straight through, rather than returning to the eye as brilliance.
This is termed a window (right). In well-cut gems,
most light returns as brilliance (left). Brilliant areas are
those showing bright reflections. Extinction is used to describe
dark areas where little or no light returns to the eye.
Photo: Wimon Manorotkul
Symmetry. Like any finely-crafted product, well-cut gems display an obvious attention to detail. A failure to take proper care evidences itself in a number of ways, including the following:
Finish. Lack of care in the finish department is less of a problem than the major symmetry defects above, because it can usually be corrected by simple repolishing. Finish defects include:
Scintillation (sparkle’). This is an important factor in faceted stones. A gem cut with a smooth, cone-shaped pavilion could display full brilliance, but would lack scintillation. Thus the use of small facets to create sparkle as the gem, light or eye is moved. In general, large gems require more facets; small gems should have less, for tiny reflections cannot be individually distinguished by the eye (resulting in a blurred appearance).
Dispersion (fire’). This involves splitting of white light into its spectral colors as it passes through non-parallel surfaces (such as a prism). While diamonds show this property to great effect, with most colored stones, their dispersion is too low and the masking effect of the rich body color so high, that it is not generally a factor. Exceptions are gems such as demantoid garnet, grandite garnet from Mali and sphene. In gems such as these, a weaker body color can actually be desirable, making the fire more visible.
Summing up cut. While
these guidelines may be useful, one must not become a slave to
them. In essence, the cut should display the gem’s beauty
to best advantage, while not presenting setting or durability problems. If
the gem is beautifully cut, things such as depth percentage or
length-to-width ratio matter not one bit. What works, works. The
eye, the mind and the heart are the final arbitrators, not numbers.
One final note about cut. The most expensive colored gems (particularly colored diamonds and rubies) often feature misshapen proportions and symmetry. This is because the value of the material is so high that the cutter strives to save every point in weight.(back to top)
Carat Weight: The Fourth C
Weight in gems is calculated in metric carats, where five carats equal one gram. Generally, as a gem’s weight increases, so does the per-carat price. Such a relationship has long been known, and was first quantified by Villafane in 1572, for diamonds. Today it is most commonly referred to as the Indian Law’ or Tavernier’s Law’, and works as follows:
Wt2 x C = price per stone
Weight of gem = 5 ct (Wt)
Cost of a 1-ct gem of equal quality = $1000 (C)
5 x 5 x 1000 = $25,000 total stone price
The following shows how the price of a gem might increase with this formula applied using a $1000/ct base price.
Total stone price
Unfortunately, things were not so simple, even for diamonds in the time of Tavernier. The law could not accurately predict the price of diamond below 1 ct, and there were also problems with exceptionally large stones. But it does give a general idea of how prices increase with size. (back to top)
Color coverage: The Fifth C
With gems, we are not dealing with opaque, matt-finish objects of uniform color. Thus it is not enough to simply describe hue position, saturation and darkness. We must also describe the color coverage, scintillation and dispersion.
Differences in proportions, inclusions, transparency, fluorescence, cutting, zoning and pleochroism can produce vast differences in the color coverage of a gem, particularly faceted stones. A gem with a high degree of color coverage is one in which color of high saturation is seen across a large portion of its face in normal viewing positions. Tiny light-scattering inclusions, such as rutile silk, can actually improve coverage, and thus appearance, by scattering light into areas it would not otherwise strike. The end effect is to give the gem a warm, velvety appearance (Kashmir sapphires are famous for this). Red fluorescence in ruby boosts this still further.
|Color coverage can be influenced by a variety of factors, including proportions, fluorescence and inclusions. The round Burmese red spinel at left is strongly fluorescent and the red emission adds extra power to the red body color, covering up extinction. With the fine emerald-cut Kashmir sapphire pictured at right, color coverage is improved by the presence of tiny needlelike inclusions, which scatter light across the stone, thus reducing extinction. This is what gives Kashmir sapphires their incomparable velvety color. Note that both of these gems have colors which are highly saturate, making them highly desirable. Photos: Wimon Manorotkul, John McLean|
cutting is vital to maximize color coverage. Gems cut too shallow
permit only short light paths, thus reducing saturation in many areas.
Such areas are termed windows. Those cut too deep allow light
to exit the sides, creating dark or black areas termed extinction.
Areas which allow total internal reflection will display the most
highly saturated colors. These areas are termed brilliance.
Color zoning can also reduce color coverage. Ideally, no zoning or unevenness should be present.
Pleochroism is noticeable face-up in some gems, such as some tourmalines and iolite. It typically appears as two areas of lower intensity and/or slightly different hue on opposite sides of the stone.
||The effects of pleochroism can clearly be seen in this oval green tourmaline. Along the vertical axis, a bluish green color is seen, while along the horizontal axis, the color is yellowish green. This is a product of the doubly refractive nature of tourmaline. Photo: Wimon Manorotkul|
In summary, a top-quality gem would display the hue of maximum saturation across a large percentage of its surface in all viewing positions. The closer a gem approaches this ideal, the better its color coverage. (back to top)
Colored Stone Grading: Art or Science?
Among the problems of existing colored stone grading systems is that the model chosen is based on diamond. While diamond does share a number of quality factors with colored stones, others are partly or wholly inappropriate. For example, beauty in diamond is largely a function of the material’s brilliance and dispersion (fire’). Any inclusions which alter the path of light could be detrimental to a diamond’s appearance. Perfect clarity is thus the ideal. As described above, perfect clarity is not necessarily the ideal for ruby and sapphire. While fractures and most other inclusions do have a detrimental effect on appearance and durability, small quantities of finely dispersed inclusions (such as exsolved rutile silk) can actually improve a richly colored gem’s appearance. The watchword here is small; too much silk decreases transparency by scattering, reducing color saturation, and thus producing a more grayish color.
Fine precious stones are comparable to great works of art. Like a painting, to appreciate it, one must view the whole, not just the parts. (back to top)
Precious vs. Semi-Precious
In former times, gems were often divided into precious and semi-precious categories. The precious stones were:
Semi-precious stones included everything else. While such categories were convenient, they ignored the reality that any precious stone can be cheap if the quality is low. In addition, some of the so-called semi-precious stones can cost tens of thousands of dollars per carat (Paraíba tourmaline, for example). Thus the terms precious and semi-precious have little meaning today with regard to value. Today, it is more common to separate gems into either diamonds or colored stones, rather than precious or semi-precious. (back to top)
Pricing Factors: Why They Don’t
Always Make Sense
Prices of Genuine Jewels
The prices of jewels are not stable. There is no law governing their prices, and there is no reason why these prices should not fluctuate with time and place. Each country, each nation carries its own temper. Furthermore, at one time nobles begin to sell them off and at others, to stock them. Stones are plentiful at one time and scarce at another. God grants honour to some and disgrace to others.
al-Biruni, 11th century AD
Kitab al-Jamahir fi Marifat al-Jawahir
One of the great mysteries for the gem novice is the relationship between price and quality. In a perfect world, price would directly relate to quality/weight/rarity. Unfortunately, Planet Gem is far from symmetrical. Market factors can have as much, or even greater, impact on prices as does quality. Prices are influenced by the following factors:
Just a few of the market factors that influence price include:
|Lighting can have a dramatic effect on the appearance of any colored gem. Incandescent lighting (left) is rich in red, orange and yellow wavelengths and thus pushes an object’s color in that direction. In contrast, skylight (right) is more balanced, pushing the color in the opposite direction. (Illustration: Minolta)|
Influence of lighting on color
With any colored gemstone, the color seen depends on the light source used to illuminate it. Over time, gem dealers have come to rely on skylight for their gem buying. Its major advantage is its strength, which ruthlessly reveals flaws. The quantity of light coming through even a modest-sized window is far greater than even the strongest, color-balanced fluorescent tube (or tubes). Another factor appears to be the large radiating area, when compared with the most artificial lights.
Latitude may also affect a stone’s color, simply because skylight is stronger in the tropics. As a result, gems bought in the tropics will appear slightly darker when taken to more temperate climes. It is a slight, but nevertheless, noticeable difference. Surprisingly, north skylight (or south skylight in the southern hemisphere) is actually stronger on cloudy days.
Another factor is the Purkinje shift. In bright light, the eye is more sensitive to red; conversely, in dim light the eye is more sensitive to blue-violet light. Thus the color of blue sapphires would be slightly enhanced in dim lighting.
The question of north skylight. North daylight (skylight, as opposed to direct sunlight) has become the standard, because it produces the least glare, but blind adherence to such gemological dogma is just as bad as blind adherence to religious dogma. If you live north of the Tropic of Cancer (Europe, North America, Japan, China, etc.), north skylight will provide the least glare year round, because the sun always passes through the southern portion of the sky. This is especially true the farther north one goes. The opposite holds true for those who reside south of the Tropic of Capricorn (in the southern hemisphere), where the least glare is found using south skylight. Natural light is not constant in spectral composition, but varies according to latitude, time of day, cloud and pollution conditions and whether or not one is using direct sunlight or skylight.
we use skylight, instead. Such skylight is actually more blue early
and late in the day. Thus blue sapphires will look better at those
times. Conversely, when viewed with skylight, rubies will look best
around midday, because the skylight is less blue.
What about those who live in the tropics? If they are north of the equator, north skylight is best, except MayJuly, when south skylight is preferred. For the tropics south of the equator, south skylight is best, except from Nov.Jan., when north skylight is preferred. And if you live right on the equator, use north skylight from Oct.Feb., and south skylight from AprilAugust. During March and Sept., either north or south skylight can be used.
Time of day. Even
skylight changes throughout the day. Generally speaking, rubies
(and other red stones) look best during the midday hours. Sapphires,
in contrast, look best in the early morning or late afternoon.
If you are buying, this means that rubies should be purchased early
or late in the day, while sapphires are best bought near midday,
thereby preventing a surprise when the stone is examined under
another lighting condition.
The above is in contrast to what is often reported. While direct sunlight is far more red at sunrise and sunset, the skylight is actually more blue. Since we use skylight, not direct sunlight, to illuminate gems, blue color will be enhanced early and late in the day. Similarly, the skylight at noon is less blue, thus enhancing the color of rubies in the middle of the day.
Weather and pollution. How might clouds or pollution affect color? Heavily-polluted or cloudy skies will result in more grayish (less blue) skylight, thus improving the appearance of rubies (as opposed to sapphires).
Artificial lighting. Some type of artificial light is obviously the answer to neutralize the above factors. Many dealers today do their buying under special daylight lamps designed to simulate true north daylight, with a color temperature of approximately 50006100° Kelvin. Generally speaking, while their color balance is similar to north daylight, the fluorescent tubes used suffer from low light output. A 20-watt fluorescent daylight tube at a distance of 30 cm produces about 1000 lux of illumination, while a north-facing window in Bangkok averages 6000 lux. (back to top)
Viewing geometry & background
Gems are designed to be mounted in jewelry and viewed from predetermined angles. This is generally face-up, with the gem viewed in a 180° arc from girdle to girdle. Thus it is only logical that all quality determinations be made with the naked eye under the same viewing geometry. It is important that the gem be rotated through 360° in the girdle plane, so that its appearance is seen from all angles, just as it would be when mounted in jewelry. To ensure reproducibility and repeatability, a standardized light source against a standardized, neutral background (white is best) at a standardized distance should be used. The practice in diamond grading of judging body color through the pavilion facets is madness, and has no place in colored stone grading.
When you are examining a colored gemstone, act like a cop always do a background check.
The color of the background against which a gem is examined can have a major effect on color. Which is why wily Burmese and Thai miners traditionally offer up rubies to buyers on brass plates or yellow table tops. The yellow background color counters the bluish tint commonly present in ruby, making the gems appear more red. Yellow cellophane-lined stone papers or brass tweezers serve the same purpose. Don’t be a sucker. For judging color, a plain white background is best.
Summary of quality
Again, 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 previous 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. When all the minutiae has been pored over ad infinitum, ad nauseam, take a step back and simply look at the gem. In the age of high-powered microscopes this may constitute a radical concept, but one which is necessary. (back to top)