"That which is beautiful is never too costly, nor can anyone pay too much for that which gives pleasure to all," said Abu Inan Farés, Sultan of Morocco, on completion of a beautiful building at Fez. To emphasize his delight, he refused to look at the architect's bill, but tore it up and threw the fragments into the River Fez. Sydney H. Ball, 1935, Economic Geology
I gotta
love for Angela, I love Carlotta, too. Thomas
–1948] MUCH of human activity concerns discretionary ability. The world is not composed of black and white, but of infinite shades of gray. Not fixed in space and time, these shades undergo continuous change. We are constantly called upon to make qualitative judgements. Such decisions are made daily – they are part of life – and our success in navigating life is closely tied to how we deal with these challenges. For assistance, society has developed guidelines. While such rules of thumb cannot predict the future of an individual event, if they are based upon the experiences of a large sampling of people, they have utility to the individual over the long haul. But when they are based merely upon "faith," rather than empirical methods, such beliefs constitute dogma. There is considerable evidence to suggest that many religious and cultural dogmas were at one time based on empiricism. For example, the prohibition against eating pork, widespread in Judaic and Islamic cultures, is believed to have grown out of the fact that, in desert societies, the keeping of pigs wasted precious water, and so was a selfish activity that harmed the group. But when such religions spread to wetter climes, where there was plenty of water to go around, the ban remained. Thus the problem. When empirical discovery solidifies into immobile dogma, as with the above example, the possibility of future discovery or change is ruled out, to the detriment of all. Similarly, according to the European thought extant during the time of Columbus, the earth was flat, and it was heresy to think otherwise. This is the difference between empirical beliefs, and those based upon faith alone, i.e., those based on observation and first-hand experience, rather than assumption.
Ruby & sapphire grading:
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10.1b: Vertical slice through the color solid along the yellow-violet axis. Saturation increases horizontally from the center, while lightness/darkness varies along the vertical axis. Note that the highest saturation of yellow is naturally much lighter than that for violet. A slice along the green/magenta axis would show the highest saturations to have a similar lightness. |
In his excellent article on the methods and benefits of colored stone grading, Nelson (1986) cataloged a large variety of trade objections. In this author's (RWH) opinion, the key criticisms are threefold:
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10.1c: Hue position is illustrated by the color wheel, representing a vertical slice through the color solid (the center is not shown). Mixing equal amounts of the three additive primaries (red-orange, violet, green) produces white, while equal mixtures of subtractive primaries (cyan, magenta, yellow) results in black. |
Unfortunately, the advantages to such a system are too often overlooked amidst the bluster and rhetoric. These benefits are the increased consumer confidence and thus, increased sales, which would follow adoption of such standards. Much time would also be saved by adoption of a standardized language for describing the appearance of colored gems.
The key to developing a successful colored stone grading system will be in creating a language useful for communicating the overall appearance of a gemstone. Once a gem is adequately described, it is then up to the marketplace to determine relative value. Attempts to assign relative values to each grade will succeed only if the considerations of the real marketplace are taken into account. To make these decisions, gemologists must work closely with traders.
An unfortunate paradox in the gem world (and one which is also present in many other fields) is that traders, who, by virtue of experience, are generally most qualified to judge quality, must be disqualified from doing so because of their bias. But traders must have input into the system for it to succeed.
Quality is determined by reference to the so-called 3 c's: color, clarity and cut. While these factors are well defined for diamond, no universally-accepted system exists for colored gems. The following is based on the author's own extensive experience.
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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 |
To the color scientist, given an opaque, matt-finished object, there are three dimensions to color:
Figure 10.1 is a simplified illustration of the three dimensions of color.
With gems, we are not dealing with opaque, matt-finish object 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.
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 |
| Color zoning can also be influenced by color zoning, an unevenness of color. The oval sapphire above shows moderate color zoning. Moderate to severe color zoning does impact quality, and thus price. Color zoning is always judged in the face-up position, in an 180° arc from girdle to girdle, with the gem rotated through 360°. Color irregularities visible only through the pavilion generally do not impact value. Photo: Wimon Manorotkul |  |
Clarity is judged by reference to inclusions. Magnification can be used to locate inclusions, but with the exception of inclusions which might affect durability, only those visible to the naked eye should influence the final grade.
Background checks |
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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.
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There are two key factors in judging clarity. These are:
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 ruby and sapphire, 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. [2] 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. [3]
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 aspect of quality analysis.
Evaluation of cut involves five major factors:
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 ruby and sapphire, ovals and cushions are the norm. Rounds and emerald shapes are more rare, and so receive a premium from about 10–20% above the oval price. Pears and marquises are less desirable, and so trade about 10–20% 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.
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.
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 |
The faceted cut for ruby and sapphire is 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.
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:
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:
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 mounting 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.
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.
Figure 10.3 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) |
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. [4] 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.
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.
What about those who live in the tropics? If they are north of the equator, north skylight is best, except May-July, 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 April-August. During March and Sept., either north or south skylight can be used.
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 (Newman, 1994, p. 38). 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.
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).
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Above Left. The Buddhist temple at Swayambunath, Nepal, silhouetted against a deep blue sky. It is obvious that such skylight would enhance the appearance of blue stones. Above Right. Fog in Sri Lanka's central highlands. The high moisture content gives the light a grayish cast. Right. Sunset on Sri Lanka's western coast. While such sunlight could easily enhance the color of red and yellow stones, it should be noted that direct sunlight is rarely used for examining gems. (Photos © R.W. Hughes) |
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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 5000–6100°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.
The answer appears to be short-arc xenon lamps. While rather expensive (compared to fluorescent lamps), they have a continuous output (like daylight), 6000°K color temperature, and produce illumination levels comparable to north daylight.
For an excellent summary of the entire lighting question, see Sersen & Hopkins (1989) and Sersen (1990), from which the above is derived.
Figure 10.5 When
grading gems, viewing geometry, background and controlled lighting
are crucial. The woman above is sorting sapphire rough from
Australia. (Photo: Great Northern) |
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. [5]
The appearance of a colored gem is a combination of many separate factors, each of which is related to, and affect, 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.
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.
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 Ma'rifat al-Jawahir
One of the great mysteries for novices 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:
Generally, as a gem's weight increases, so does the per carat price. This is shown in Figure 1.6.
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Figure 10.6 Graph representing the relationship between price and quality/weight/rarity. Note that this is not a linear relationship. Price increases more quickly as quality/weight/rarity increases. |
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 (Lenzen, 1970):
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| Weight of gem | = 5 ct (Wt) |
| Cost of a 1-ct gem of equal quality | = $1000 (C) |
| Calculation: 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.
| Weight | Total stone price |
| 1 ct | $1000 |
| 2 ct | $4000 |
| 3 ct | $9000 |
| 4 ct | $16,000 |
| 5 ct | $25,000 |
| 10
ct |
$100,000 |
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.
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Just a few of the market factors that influence price include:
For a generalized list of ruby and sapphire prices, see "Ruby & sapphire prices", p. 491.
This page is http://www.ruby-sapphire.com/r-s-bk-quality.htm
v. 1.1
11 August, 2008
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the
case of many gems, including ruby and sapphire, psychological
(but all too real) price jumps occur at certain weights.
For example, a 0.99-ct ruby might be worth significantly
less than one which weighs 1.05 ct. The 1.05 ruby would
be worth more than one which weighed exactly 1.00 ct,
as repolishing a 1.00-ct stone (or weighing it on someone
else's scale) might send it below the important
1-ct barrier. Similar psychological weight hurdles are
found at the 2, 5, 10, 20, 50 and 100-ct levels.
