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.
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
Color: The First C
For a colored stone (any gem other than diamond), color is the most important factor in determining quality.
- Hue position
- Saturation (intensity)
- Tone (lightness or darkness)
For colored gemstones, there is also a fourth factor:
Right: Hue position describes the position of a color on a color wheel. Illustration © R.W. Hughes
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.
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.
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.
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.
Cut (‘make’): The Third C
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.
- Cutting style
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 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.
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.
- Depth percentage: In attempting to quantify a gem’s proportions, reference is often made to depth percentage. This is calculated by taking the depth and dividing it by the girdle diameter (or average diameter, in the case of non-round stones). The acceptable range is generally 60–80%.
- Length-to-width ratio: Another measurement that is used for non-round stones is the length-to-width ratio. Overly narrow or wide gems of certain shapes are generally not desirable.
- Asymmetrical girdle outline
- Off-center culet or keel line
- Off-center table facet
- Overly narrow/wide shoulders
(pears and heart shapes)
- Overly narrow/deep cleft (heart shapes)
- Overly thick/thin girdle
- Poor crown/pavilion alignment
- Table not parallel to girdle plane
- Wavy girdle
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:
- Facets do not meet at a point
- Misshapen facets
- Rounded facet junctions
- Poor polish (obvious polishing marks or scratches)
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
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
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 zoning can also reduce color coverage. Ideally, no zoning or unevenness should be present.
Color 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
The term “enhancement” is a treatment or process other than cutting and polishing that improves the appearance (color/clarity/phenomena), durability, value or availability of a gemstone. In today’s gem marketplace, many gemstones have been enhanced by a variety of methods. Such processes may range from simple heating (such as with tanzanite) to high-tech irradiation (such as blue topaz).
Gems which have not been subject to enhancement are generally worth more than those of the same quality which have been enhanced. For more on enhancements, click here. Because enhancements affect value, it is vital that buyers be provided with full enhancement information when considering the purchase of any gem.
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.
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.
Pricing Factors: Why They Don’t Always Make Sense
Prices of Genuine Jewels
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:
- Quality: Better qualities are more rare than lower qualities of the same size (see previous section).
- Weight: Bigger stones are more rare, and so more expensive per carat than the same quality of a smaller size.
- Market factors: This is the great intangible. Market factors can dramatically affect price.
Just a few of the market factors that influence price include:
- Market supply vs. demand: Items which are plentiful and/or in low demand will be cheaper than those which are rare and/or in high demand.
- Financial situation of the seller: Sellers who need money will obviously be more flexible on price. Similarly, those who are not in need are less willing to reduce their price.
- Seller’s business overhead: Prices can vary dramatically depending on the seller’s overhead. A cup of coffee purchased by a street vendor may cost only a few cents; the same cup of coffee at a 5-star hotel in the same city may cost 10–20 times more, due to the hotel’s higher overhead.
- Buyer’s financial situation: Buyers whose businesses are prospering are often willing to pay higher prices.
- Personal situation surrounding the sale: The author has seen buyers pay above-average prices for goods for a variety of reasons. These have ranged from trying to impress one’s girlfriend, to buying something simply to prevent a competitor from purchasing the same goods.
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.
Typically 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 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.
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 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.
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.
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