CS ASSINGMENT #8

The Economics of Color and Cut

A cutter's primary goal is to produce a stone with the highest possible value. Often, that means getting the best weight return from the rough. To produce an attractive stone, the cutter must often balance economics and art. This is especially true when the rough is expensive and rare. Some people might think a cutter would try to get the largest possible finished stone from a large piece of dark red garnet rough, but that's often not the case. Experienced cutters know that large stones cut from dark material appear almost black, and customers want garnets that look red face-up. It would be better to cut several small gems from the same piece of rough. The small gems would be much more attractive. Cut variations are accepted in gems fashioned from flattened rough, like some rubies. The flatness of the rough limits the possible depth of the finished stone's crown and pavilion. People in the trade recognize that cutting variations in this type of material represent the best possible balance of fine color and weight retention. With expensive stones, compromises made to save weight reflect a cutter's ability to produce usable faceted stones out of very challenging materials. If the rough is very valuable, loss of weight equals loss of money. Cutters can't afford to lose gem weight by cutting to correct proportions. And some very fine rough can produce excellent color even if the stone's proportions are not perfect.

Cut

A master cutter unlocks the beauty within a 100-ct. rough peridot crystal from Pakistan, resulting in a 10-ct. cushion cut. Testifying to the cutter's skill, sharp reflections ripple across the gem's perfectly positioned facets as the stone moves. The gem's entire surface is a beautiful, rich green, with no obvious dark or light areas. Cutters work with what nature offers, and a cutter has to examine a rough crystal very closely before making the first cut. Every piece of rough has its own unique pattern of color distribution, which is the amount of color and its location in the crystal. The blue color in a rough sapphire, for example, might be located only at the tips of the crystal. An evenly colored piece of rough can produce finished gems of different colors, depending on their sizes and proportions. The color of a gem cut from light-colored pink topaz rough, for example, is attractive enough only if the gem is above a certain size. The cutter might decide to cut one large, deep stone rather than a few better-proportioned—but more lightly colored—smaller stones. Skilled cutters also consider color that's arranged in layers parallel to the crystal faces or near the crystal's surface. They try to cut the finished stone to show the best possible color through the crown. To accomplish this, they can arrange colored layers so they're parallel to the faceted gem's girdle, or locate strong color in the culet. Correct proportions can maximize brilliance and color, but as you'll see, it isn't economically possible or desirable to cut every piece of rough to perfect proportions.

Color Change

As you learned in Assignment 4, color change is a noticeable change in gem color that occurs under different types of lighting. Chromium and vanadium are the coloring agents usually responsible for color change in gems. Color change occurs in alexandrite chrysoberyl, color-change sapphire, and color-change garnet because these gems transmit the red and green colors of the spectrum almost equally. These gems show different bodycolors, depending on the type of light they're viewed under. For example, an alexandrite appears red under incandescent light, which is rich in red. Under many fluorescent light sources, which are richer in blue and green, it appears green. Although the hue is different under each light source, the tone and saturation are usually similar. When you judge color-change gems, examine them under daylight-equivalent light first, and then switch to incandescent. Consider the extent and intensity of the color change. In alexandrite, a distinct green to red change is very highly valued.

Pleochroism

As you learned in Assignment 4, doubly refractive stones can show different colors through the crown. In most directions within certain transparent, colored, doubly refractive gems, light is split into two separate rays, which might show as different bodycolors, depending on the cutting orientation and viewing angle. This is called pleochroism. In stones like andalusite, iolite, and tanzanite, pleochroism is visible to the unaided eye. Andalusite, for example, can be cut to show its unique mix of yellow-green and orange pleochroic colors through the crown. And tanzanite can show a more bluish or more violet face-up color, depending on cutting orientation. For some pleochroic gems—like ruby, tourmaline, and sapphire—some pleochroic colors are less desirable than others, so the rough must be oriented properly before cutting to show the best face-up color in the fashioned gem. In ruby and sapphire, fortunately, the orientation that gives the most favorable color is also the orientation that gives the maximum weight yield. Blue sapphire, for example, shows its best pleochroic color if the table facet is cut at right angles to the crystal's length. If it's cut with the table facet parallel to the crystal's length, it might show a less desirable greenish blue face-up color. Even if the stone is correctly oriented, some pleochroism will be visible through the table facet due to the reflection of light rays within the gem. Some singly refractive stones—spinel, for instance—can show a second color face-up. This isn't pleochroism. It happens when light that's reflected within the stone interacts with light that's transmitted through it. If you check with a dichroscope, you'll see only one color at a time. The sapphire (left) shows both blue and green pleochroic colors. The pure blue pleochroic color is much more highly valued than the greenish blue one. Some singly refractive gems might appear pleochroic, but they're not. The yellowish orange and strong orange hues in the 19.24-ct. Malaya garnet (right) are due to reflections within the gem, not pleochroism. The dichroscope would show only a single color.

Using GIA Terms to Describe Gemstone Color

Because gem color descriptions are so varied and subject to individual perception, GIA developed the GIA Colored Stone Grading System to help judge an individual stone's color. This system blends color science and practical trade information to help you place a stone on a scale from the "most preferred" to the "least preferred" color for its species or variety. The GIA color system uses 31 hue names. To describe a gem's color, it combines basic hue names (red, orange, yellow, green, blue, violet, and purple) with any modifying hue names that might be necessary. GIA developed a shorthand system for this notation. The dominant—or stronger—hue is capitalized. For example, red modified by a slight amount of purple becomes slpR (slightly purplish red). If the purple is strong, the designation would be stpR (strongly purplish red). Where the stone shows approximately equal amounts of purple and red, the shorthand is RP/PR (red-purple or purple-red) and both hues are written as capitals. If purple dominates, it's written as rP (reddish purple). Tone and saturation come next in the notation. The system divides tone into 11 levels ranging from colorless (transparent) or white (opaque)—designated as 0—through increasingly darker grays to black—designated as 10. In practice, only levels 2 (very light) through 8 (very dark) apply to transparent colored stones. Tones that are any darker or any lighter are nearly impossible for the human eye to detect. Three standard tone levels are 3 (light), 5 (medium), and 7 (dark). With a little practice you can estimate the values between them. The saturation scale has six levels. If the hue has any brownish or grayish components, saturation is 3 or lower. If brown or gray appear distinctly, saturation is 1 or 2. If there's only a slight hint of brown or gray, then saturation is around 3. At level 4 and above, there's no trace of brown or gray, and the hue's strength increases through levels 4, 5, and 6. A saturation rating of 4 or higher makes an attractive colored stone, all other things being equal. For most colored stones, the most valuable colors tend to fall within medium-light to medium-dark tone (4, 5, 6) and moderately strong to vivid saturation (4, 5, 6). Putting all the notations together, you might end up with a gemstone described like this: vslgB8/3 (very slightly greenish blue—with blue the dominant color—with very dark tone and very slightly grayish saturation). The advantage of a description like this is that it means the same thing to everyone. This makes it an improvement over terms like "sky blue" or "midnight blue," which could be open to a variety of interpretations.

Sidebar: Using GIA's Color Descriptions in the Marketplace

Dealers often have a bewildering array of grades and prices that make it difficult to know what quality of color to expect in stones at various market levels. A good example of this is blue sapphire, which comes in a range of qualities and price levels. When you picture the market categories for sapphire—or any gem—imagine a pyramid, with commercial-quality gems at the base, middle-market stones in the center, and fine-quality stones at the peak. The ranges of hue, tone, and saturation narrow as you get closer to the top of the pyramid. The number of available stones also narrows dramatically with each increase in quality. At the commercial level, you can expect to see two broad types of blue sapphire: very dark-toned stones of inky or blue-black appearance and light-toned stones with low saturation and a grayish appearance. Dark material is much easier to match and more consistent in appearance. It's good for large orders or catalog sales, where uniformity is important. Dark-toned stones hide their hues and their clarity. Dark-toned sapphires can conceal color zoning and concentrations of silk. Cut and proportion won't matter as much as the fact that not much light is being transmitted through the stone. For the same reason, the quality of polish on the pavilion matters less than the quality of polish on the crown. Brilliance is low in dark-toned sapphire. In practice, lighter sapphires are harder to match, with the result that parcels might differ slightly in appearance. In other words, you might request the same size, shape, and price per carat, but receive goods with slightly different hue, tone, or clarity. This can be a problem for a client who expects uniform goods. The GIA Colored Stone Grading System provides a systematic, repeatable method for noting the quality of colored stones. In commercial blue sapphire, expect to see the widest hue range from violet through very strongly greenish blue (V to vstgB). Expect to see a range of tone from 2 through 8, but less of a saturation range. Saturation from 1 to 4 would be typical. Dark-toned commercial sapphire might be noted like this under the GIA system: V8/1, bV8/3, vB7/3, B6/2, vslgB8/3, gB8/1, vstgB8/3. Light-toned commercial sapphire might be noted as V2/2, bV2/3, vB3/3, B3/1, or vslgB2/2. Stones at this commercial level form the base of an imaginary quality pyramid. Middle-market blue sapphires display a narrower range of hues than commercial sapphires. They usually range from violet to very slightly greenish blue (V to vslgB). Their ranges of tone (from 3 through 7) and saturation (3 through 5) are usually also narrower. Some examples of this category might be bV6/4, vB4/4, B5/4, and vslgB7/5. The finest blue sapphire is at the top of the quality pyramid. Hue range is very narrow, from violetish blue to pure blue (vB to B), with pure blue the preferred hue. Tone (4 through 6) and saturation (5 to 6) also occupy narrow ranges. Examples might be vB6/5 and B6/6. Remember that these categories are only suggestions and that they might change over time with supply and demand or discovery of new sources.

Fluorescence

Fluorescence happens when a material emits visible light—glows—when it's exposed to invisible ultraviolet (UV) radiation. Sunlight and fluorescent lamps emit some UV radiation, and many colored stones fluoresce in response to it. Fluorescence adds an extra glow to red spinel's appearance. It also boosts the red bodycolor of chromium-rich rubies, sometimes enough to counteract extinction. But rubies from some sources are rich in iron, and the iron content inhibits the fluorescent effect. In general, fluorescent rubies are more highly valued than non-fluorescent ones, even though one side effect of fluorescence is a loss of brilliance.

Sidebar: Color Comparison Systems As you've learned, a slight difference in color can have a major impact on a gem's perceived value. Over the years, appraisers and gem laboratory professionals have developed systems for standardizing color descriptions of faceted gems. Several have gained a degree of industry acceptance. As far back as the 1700s and 1800s, scientists and philosophers had considered approaches to representing colors in a three-dimensional way, but it wasn't until 1905 that Albert H. Munsell developed the first practical color description system. Munsell, a Massachusetts college arts professor, published his book A Color Notation. The book provided a scientific approach to visually identifying and matching color. Munsell wanted a repeatable and easy-to-understand system for artists and scientists to use when communicating color. The first Munsell Atlas of Color had 10 hues and was introduced in 1915. It was expanded to 20 hues in 1929 and to 40 in 1950. The atlas consists of 40 pages of printed color chips with more than 1,500 colors sorted by hue, value (tone), and chroma (saturation). Today, the system is widely used in business and across many disciplines. The Munsell system is the foundation of many gem industry color communication and appraisal tools. It's also the basis for the GIA Color Description System taught in this course and in the Colored Stone Grading Lab Class. GemDialogueTM GemDialogue was originally developed in 1983 by Howard Rubin and Gail Brett Levine as an affordable appraisal tool. It uses 21 transparent color scale charts to display 10 saturation levels, or "zones," for 44 hues. The user estimates tone by overlaying a black-to-gray or brown-to-light-brown scale onto a color chart. The system is available in the United States through Gemworld and the National Association of Jewelry Appraisers.

GemeWizardTM GemeWizard is a color communication and gem trading software developed by Menahem Sevdermish of Advanced Quality (Ramat Gan, Israel). First introduced in 2003, GemeWizard is designed to help industry professionals communicate gem color, manage their inventories, and special-order gems for their customers. GemeWizard uses the same color language as the colored stone grading system taught in GIA's Graduate Gemology program. Its system was developed with extensive feedback from gem industry professionals from a database consisting of thousands of digital gem images. The basic system displays gem colors in a 31-hue rectangle—the "GemeSquare." Clicking on a hue allows the user to view the tone and saturation levels for that hue. The system encompasses a total of 1,146 colors, and users can view gem colors in up to 15 different cutting styles. GemeWizard was originally designed for laptop or desktop computers, but in 2017 a new version was released as an app for a range of mobile devices, including smartphones. Gemworld's World of ColorTM Designed and developed by Richard Drucker and Thomas Tashey in 2014, World of Color is a conveniently sized printed and spiral-bound book showing 40 gem hues with their corresponding tone and saturation values, for a total of 1,400 possible colors. World of Color layout and terminology is based on the Munsell color system. Each page has a transparent overlay that integrates the color-naming charts of the Inter-Society Color Council (ISCC) and the National Bureau of Standards (NBS). On each page, the possible hue positions are represented by colored ovals. The system comes with a transparent colorless plastic "crown" that the user can place over the selected hue position to help provide a three-dimensional representation of the printed color, which can then be compared with the gem being examined.

Clarity

Inclusions that disrupt the path of light through a faceted gem reduce brilliance and color intensity. But some inclusions can have a positive effect on appearance. Fine inclusions can scatter light within a faceted stone and make extinction much less noticeable. Stones with a slightly diffused, or "sleepy," appearance, like some sapphires, show this effect. Inclusions can also give a gem its color. Bright particles of green mica give aventurine quartz its green bodycolor, and inclusions of brownish hematite add depth to the color of some yellow sapphires. Some colored gems contain a multitude of tiny inclusions that scatter light. This gives them a soft appearance. Dealers often describe gems with this property—like this suite of corundum—as "sleepy." A sleepy gem might have reduced color and brilliance, but it often displays much less extinction than a completely transparent one. Short, stubby, reddish brown hematite platelets give the yellow sapphire (left) a sleepy appearance. They might also deepen its yellow hue. Long, slender rutile needles can cause the sleepy appearance in blue sapphire (right).

Hue, Tone, Saturation, and Value

Judging colored stones is different from judging colorless to near-colorless diamonds, where absence of color is a defining virtue. Diamond graders focus on clarity—the presence or absence of inclusions—along with the precision and quality of the cut and the presence of even slight color. Fancy-color diamonds are more like colored stones in that they're judged primarily on their color, and issues of clarity or perfection of cut are secondary. Like colored diamonds, vibrant color is the chief glory of most colored gemstones. Because individual gemstone species have characteristic color ranges, it's important to know a gem's identity before you grade it. The color of a particular stone should be judged against other stones of the same species or variety, so there are consistent standards for hue, tone, and saturation. It's no good to judge a stone as a poor ruby when it's actually a pretty good garnet. Before you look at the color of a gem, clean the stone thoroughly with a gem cloth. Make sure its surfaces are free from fingerprints and dust. Pick the stone up by its girdle with your tweezers, or place it on a neutrally colored—white or gray—surface. Evaluate the stone's overall tone—its level of lightness or darkness. To judge tone, consider the stone as a whole. Look at it in the face-up position, and "average" all the different areas you see. Look for a general impression: Does the stone appear light, medium, or dark? Next, assess the bodycolor and identify the hue. Although this is not always quite as easy as it sounds, ignore surface reflections, areas of extinction, and minor windows. Here's a handy tip: If the stone isn't mounted, turn it onto its table—on a neutral background—and look at the color through the pavilion. This eliminates some of the reflection and scintillation, and makes it easier to see color. Now look at the stone face-up, and look for the same color you saw on the pavilion. Look for the representative color of the stone apart from the darker areas caused by extinction and the lighter areas caused by reflection or windowing. Now that you've identified the stone's hue and tone, consider the saturation, or strength, of the hue. If it's a cool color like green, blue, violet, or purple, does it look grayish? If it's a warm color like red, orange, or yellow, does it look brownish? If you can't see any gray or brown, then the saturation is moderately strong or higher. Low saturation and light tone generally give a colored stone low value. The small calibrated aquamarines used in mass-market jewelry might be almost colorless. The least expensive grades of amethyst are a pale lilac hue, which is a low saturation of purple. Compared to vibrant purple top-grade amethysts, they can look like an altogether different gem species. Hues at opposite extremes of the tone range—very light or very dark—commonly don't reach levels of saturation as high as those in medium tones. Dark-toned stones might have high saturation, but they absorb light so strongly that you can't see the saturation under ordinary lighting conditions. Although you can appreciate the color of gems like these only by shining a light through them, you should grade them under normal lighting. It's difficult for your eye to detect hues of low saturation. Weakly saturated hues in inky blue sapphires, dark red garnets, and dark green tourmalines are hard to detect because of dark tone. Low saturation and dark to very dark tone almost always indicate low value. Size also matters for some gems. Larger stones can appear more saturated than smaller stones cut from the same rough. Aquamarine rough that produces superb stones of 5.00 cts. and above might produce pale, near-colorless smaller stones. Gems such as imperial topaz, morganite (pink beryl), kunzite, and aquamarine achieve the best, most saturated hues only in larger sizes. Some stones have intensely saturated color that would be too dark in large sizes, so they're better as smaller stones. Emeralds from Sandawana, Zimbabwe, make excellent small calibrated squares and rounds because they show color even down to 2 mm in size. Stones above 1.00 ct. are comparatively rare, and they're often too dark in tone to be considered really fine. Blue sapphires from Pailin, Cambodia, also display their strongly saturated color best in smaller sizes, typically below 2.00 cts. Small rhodolite garnets can be a vivid, rich purplish red. In general, a gem becomes more valuable as its saturation increases. Value increases most strongly between moderately strong (4) and vivid (6) saturation, when a gem's tone is medium (5) to medium dark (6). As with fancy-color diamonds like these, color is the most important judgment factor for many colored gems. Dealers overlook a colored gemstone's proportions and clarity if its color is attractive.

How does a gemstone's cut influence its face-up color? What causes windows and extinction? How do color zoning and pleochroism influence a gem's color or appearance? How do clarity and fluorescence affect a stone's color?

Just because a rough crystal looks promising when it comes out of the ground or the streambed doesn't mean it will end up as a dazzling gem. Many things can get in the way of this, including the gem's own internal structure and clarity characteristics. Add to this the challenge of making just the right cutting decisions, and you'll realize that a richly colored finished gem is no accident—it's all about the way the gem and the light interact.

Using Your Color Knowledge

Many factors go into your assessment of a gemstone's color. Is the tone light, medium, or dark? Is its saturation weak, moderate, or strong? Does the color fall within a range preferred by the gem trade? Rough is often heavily color zoned, so a stone might show areas of different face-up colors. The color you see face-up is often the result of the cutter's ability to use very localized areas of color within the rough. Most of the defects in colored stone proportions—for example, windows and extinction—are compromises made by the cutter to produce economically viable stones. Some stones, such as sapphire, ruby, and tanzanite, show pleochroism. Correct orientation of these stones can yield the best possible face-up color. The hue and saturation ranges of many gemstone species overlap. It's possible to offer some gems as lower-cost alternatives for the Big 3 and other expensive gemstones that have similar colors. A fine 5-ct. tanzanite can resemble the finest Kashmir sapphire, but it can wholesale for hundreds of dollars per carat instead of tens of thousands of dollars per carat. A suite of emerald jewelry, out of the reach of all but the wealthiest consumers, can be reproduced using tsavorite or chrome tourmaline for far less than the original piece. If a showy ruby pendant is beyond your customer's budget, try rubellite tourmaline, or even rhodolite garnet. Try to examine the gems you buy or grade under a consistent light source. If you're on a buying trip to Bangkok for a customer in New York, think of the lighting conditions you'll examine the stones under. Remember that natural light varies widely, and rely on your own color samples for grading. In this assignment, you've seen that color is a vital part of gemstone value. In the next assignment, you'll learn how the intricacies of cut go hand in hand with color. Think of a gemstone as the sum of its parts. Each part makes a contribution to your assessment of the gem. The more the stone "tells" you about its features, the better your judgment of its quality, beauty, and value.

Color Zoning

Often, a rough gem's color isn't well distributed and the cutter has to compromise to capture whatever color is present. Blue sapphire and amethyst often have areas of strong color, usually parallel to their crystal faces. This is called color zoning, and it's caused by variations in conditions and coloring agents that occur during crystal growth. Color zoning appears as areas of uneven coloration or different colors seen from the same viewing direction. Dark-toned sapphires from some types of deposits often contain very strong, angled color zoning, made up of distinct bands of different colors or different color intensities. You'll frequently see strong color alternating with almost colorless bands. The tone of these sapphires is usually dark enough to conceal the color zoning. Skilled cutting can reduce the effects of color zoning. In amethyst, prominent zoning can be oriented so the bands of color run parallel to the girdle plane. Strong color zoning in amethyst—even in the top grades—is usually acceptable as long as it can't be seen through the gem's crown. In color-zoned crystals, the cutter can concentrate the color in a small area, usually as close to the culet as possible. Even if the rest of the stone is much lighter or even colorless, this gives remarkably even face-up color. If there's face-up color zoning that the cutter was unable to conceal, it's called unintended color zoning and considered a clarity characteristic. Sometimes, the concentration of trace elements changes abruptly as a gem grows. In some gems, this change of trace-element chemistry results in a change of color during growth. This gives the stone two or more distinct color zones. If the contrast in color is striking, the cutter can cut the gem to make the most of this feature. Gems like ametrine and bicolor tourmaline can be deliberately fashioned to show color zoning. The GIA system describes this as intended color zoning because it's part of the finished stone's unique appeal. With intended color zoning, cutters take advantage of strong color zones to produce striking gems that display two or more contrasting colors. This unusual 2.11-ct. corundum from Vietnam (top) shows both blue and pink colors in a single gem. The 19.95-ct. ametrine (bottom) combines amethyst and citrine colors. The cutter was unable to conceal the strong color zoning in these sapphires. As a result, you can see strong color bands through their crowns. This is called unintended color zoning.

Lighting and Consistency

One of the most important aspects of buying and grading colored stones is the type of lighting you use. For consistency, you should view colored stones under daylight-equivalent fluorescent lighting whenever possible. Don't buy stones under lighting that's too red or too blue. Incandescent lighting with its high yellow and red content will flatter stones that transmit these colors. Ruby, red garnet, spinel, fire opal, and red tourmaline all look better under incandescent light. If you use an incandescent light to look at blue sapphires—which absorb red light and transmit blue light—they won't look their best. If you're buying an important stone, check it under a variety of light sources. When you grade a stone for sale or purchase, try to see it through the eyes of its final judge, the consumer. A sapphire that looks slightly dark in the strong light of a city like Bangkok or Nairobi—both close to the equator—will probably look inky and black in the less intense light of northern cities like New York or London. It might be difficult for you to detect slight color variations when you're out of your own familiar surroundings. For instance, it's hard to make distinctions in hue with pale stones like aquamarine. The color might look fine at the dealer's office, but it might have a greenish hue when you look at it under your own conditions. When you know you won't be able to control the lighting conditions, make sure you take comparison stones of known color or standardized color samples along with you. You can compare them with the new stones under the same light and make a good grading judgment. If you're buying stones for a specific customer, it's best to have a number of sample stones. Get your customer to approve one, leave one at your office, and take one or two with you when you go on a buying trip. You might even want to leave a sample with a reliable dealer so you can conduct future transactions by mail. Your grading procedures should be consistent. If you always grade colored stones 12 inches from your light source, stay with the practice. The strength of the illumination drops quickly with distance, and this can affect your judgment of gemstone color. If you use natural lighting, try to grade at the same time each day. Make sure you follow these simple procedures. It's the best way to make sure that your grading will always have reliable and repeatable results.

After you know a gem's identity, judge its tone, and then determine its hue. These sapphires show a range of hues from yellow through reddish orange. You can sometimes get a better idea of a gem's hue, tone, and color consistency if you examine it pavilion-up. This can help decrease reflections. These 5x4-mm chrome tourmalines show a range of tone from light to dark. The broad tone range of these blue sapphires places them into three groups. The five gems on the left range from very light to medium-light in tone, the two in the center are medium to medium-dark, and the five on the right range from dark to very dark. The two center sapphires represent the most attractive combination of tone and saturation. Under normal lighting conditions, it's difficult to determine the hues of some very dark-toned gems. Shining a light through them can help (left). Inky blue sapphires are often so dark that they appear black under reflected light (right). Many gems reach strongly saturated hues only in large sizes. Small calibrated amethysts (left) rarely achieve the saturation of large free-size gems (right).

Some gems have such vivid saturation that they become too dark in larger sizes. Emeralds from Sandawana, Zimbabwe, are known for their vivid color in small sizes. Many gems display attractive colors only in large sizes, whereas others can be strongly saturated in much smaller sizes. Although it's close to 650 cts., the Brazilian kunzite (left) can't match the intense color of the much smaller rubellite (right).

GIA Color Description System

Starts on PG 76 CS Book 2

Trade Terms

The language of the colored stone market contains a sometimes confusing array of descriptive terms: "Paraíba" tourmaline, "Mozambique" garnet, "Sandawana" emerald, and "Kashmir" sapphire, to name a few. These trade terms, and many others, are widely used in the jewelry industry. Trade terms connect certain words with particular gemstone colors or geographic locations. Some trade terms describe a gem's color without hinting at its geographic origin. For instance, "chrome" tourmaline suggests a vibrant green stone of higher quality than the typical green tourmaline, and "hot pink" was used for bright pink tourmaline in the 1980s and 1990s. Source names like "Burma" and Kashmir have become synonymous with the finest corundum colors, but these terms should be used only for gems from these specific areas. Sometimes, dealers use trade terms as sales or marketing tools. In an attempt to praise the color, a seller might describe a ruby as Burma color, or a sapphire as the best Kashmir blue, even though they're not sure of the gem's place of origin. If you use terms like Kashmir to describe a fine blue sapphire, or Mogok for vivid red ruby, be sure each of these gems comes from that particular place. If you can't positively identify a gem's origin, don't use these terms. However, you should be aware that sellers often mix inexpensive gems from many different sources together for sale. For instance, dealers in Bangkok—the world's corundum marketing center—mix dark blue sapphires from Nigeria, Vietnam, China, and Thailand and offer them in the trade as "Australian." They can do this because sapphires from all of those deposits are very similar, and because their wholesale customers readily accept dark or inky blue sapphire as Australian. The use of a trade term to describe a fine ruby, sapphire, or emerald might be supported by stronger evidence than just appearance, such as a certificate of origin. Dealers in fine stones sometimes know the geographic origin and history of the gems they specialize in, but they might not be as familiar with other gems. If a dealer offers you a fine blue sapphire and describes it as "Burmese," make sure you can confirm that it actually came from that source. Ethical dealers take responsibility for the origin of the stones they sell and don't make claims they can't support. You'll learn more about trade terms in the individual stone assignments later in the course. Ask almost any dealer for Mozambique garnet and you'll get stones that look like these attractive brownish red garnets in medium to dark tones. Dealers sometimes describe vibrant pink tourmaline or sapphire as "hot pink." The term describes a highly saturated but light- to medium-toned pink gem. Some trade terms are very specific. For example, you should use the term Paraíba only for striking blue tourmalines colored by copper. This custom mounting features a 2.69-ct. Paraíba tourmaline set with diamonds.

Bodycolor

The smooth, polished facets of a fashioned stone reflect some of the light that strikes them. The light that's not reflected enters the stone. The white light is split into its spectral colors, and the gem absorbs some of the colors and transmits the rest, or returns them to the viewer. As you learned in Assignment 4, this process—where some light is reflected, some is absorbed, and some is transmitted—is called selective absorption. The transmitted wavelengths give the gem its basic color, called bodycolor. Bodycolor is a combination of hue, tone, and saturation. Hue is your first impression of a gem's basic color. It's the green of a tourmaline or the blue of a sapphire. Hue might be described as red, orange, yellow, green, blue, violet, or purple. Some color descriptions might also require a combination of color terms. For example, a ruby might have some blue color zoning that gives it a purplish face-up color, so you'd call it "purplish red." Stones that show a combination of hues face-up are usually less valuable than stones that show a single pure hue. This is the case with sapphire and ruby, for example. Pure blue sapphires are much more valuable than sapphires with green modifying hues, and orange and strong purple modifying hues are less favored in ruby than pure red. Tone is the darkness or lightness of a color. Many dark-toned stones, such as dark, "inky blue" sapphires, absorb so much light that they must be cut shallow to yield a lighter-looking stone. Saturation is a color's strength or intensity. A large part of a gemstone's value lies in the saturation of its hue. Whether an amethyst's color is a light grayish purple or a rich purple makes a big difference in its value. In many gems, like tsavorite garnet and amethyst, highly saturated hues might produce dark tones. If the tone is very dark, it might be difficult to assess the gem's hue. Other gems, like pink sapphire and pink tourmaline, can reach very saturated hues without becoming dark in tone. In some gems, it's difficult to decide whether a gem's bodycolor is the result of a highly saturated hue or dark tone. Low saturation shows up differently, depending on whether the hue is "warm" (red, orange, and yellow) or "cool" (purple, violet, blue, and green). Generally, warm-colored stones with low saturation look brownish, while cool-colored stones with low saturation look grayish. So descriptions of low saturation levels include either "brownish" or "grayish." High saturation is described as "strong" or "vivid."

Windows and Extinction

When light escapes through the back of a shallow stone, it creates a window or "see-through" area of low color saturation. If you place a shallow gemstone—or some poorly cut ones—face-up on a page and you can read text through the crown, it's a result of windowing. Shallow cutting results in loss of light. Light entering the stone's crown from above meets the pavilion facets at the wrong angle and exits the stone without being reflected back to the eye. And because the pavilion is shallow, light from below travels through the pavilion and exits through the crown. Because the light has only a short distance to travel through the stone, the color is less intense than if the stone were cut to excellent proportions. With a stone of very shallow proportions, the windowed area might dominate the stone's face-up appearance. This gives the stone a washed-out look under the table. The hue and tone might not differ much from the rest of the stone, but the saturation would be two to three levels lower. If the windowed area is 50 percent or more of the face-up area of the stone, its color becomes the stone's bodycolor. Less expensive stones are often flattened and windowed when the rough limits the depth of the stone. Often, the only way to create a good-sized finished stone is to limit the depth of the pavilion and cut it in a flattened shape. This is the case with most Thai rubies. At the other extreme are chunky stones with extremely deep pavilions, called "native cuts." They're often designed purely to get the maximum weight out of a piece of rough, regardless of color or proportion. Sometimes, this is necessary because of export restrictions imposed by the colored stone's source country. Some countries allow only cut stones to be exported. Crude cutting that doesn't remove much of the original rough can be one way to meet the legal requirements and allow the stone to be exported. Stones with pavilions that are too deep have unattractive dark areas when you view them face-up. This is known as extinction. Areas of extinction shift when you move the gem. Extinction darkens the bodycolor of a gem and can lower its quality and value. Extinction is a result of the cutting process, and all stones display some extinction at some viewing angles. Light enters the crown facets, reflects off one side of the pavilion, and escapes through the other side of the pavilion instead of through the crown. Cutters don't always try to prevent extinction. Kunzites and pale amethysts, for example, are often cut deep. The deeper cut allows stronger absorption of the light that travels through the gem. This makes the gem's color more distinct, and the resulting extinction also makes the stone appear darker. When light escapes through the pavilion in an uncontrolled way, as with windowing and extinction, it's called unplanned light leakage. When light enters through a stone's crown, reflects off the inside of the pavilion facets, and exits back through the crown to your eye—as the cutter intended—it's known as planned light leakage. Cutters sometimes fashion gems to get the maximum weight retention out of an odd-shaped piece of rough rather than to improve its color. Dealers call asymmetrical, chunky gems like this sapphire "native cuts."

Practical Color Grading What should you look for when judging a gem's color? What conditions are important for effective color grading? What steps should you take to grade a colored stone?

When you look at a colored stone, all the information you need to assess its quality is there in front of you. The gem's color, cut, clarity, and carat weight are the key indicators of its value. The first of these judgment factors—color—is the focus of this assignment. You'll learn about the rest in the next three assignments. During the color grading process, a grader asks these questions: Is the stone light, medium, or dark in tone? What is the gem's bodycolor? Is the saturation weak, moderate, strong, or vivid? Is the stone's color exceptional or poor for that gem species? Colored stone grading isn't just a matter of filling in a lab report or a worksheet. It's mostly a tool for making buying decisions. Whether a gem is a single loose stone, part of a parcel, or a centerpiece in finished jewelry, it's judged by its qualities: the beauty of its color, the precision of its cut, the presence or absence of inclusions, and its size and rarity. Whether you're an appraiser, a manufacturer looking for consistency when ordering colored stones, or a retailer trying to match a customer's existing gem, you grade colored stones every day. When you grade or match colored gems, keep conditions consistent. Use a neutral background, and examine all the gems at the same distance from the daylight-equivalent light source. If you use natural light, try to grade gems around the same time every day.

The selection of colors in which a gemstone occurs is called its color range. Each gem has a different color range, and it can be broad or narrow. For example, most peridot is yellowish green, so it has a limited color range. Tourmaline, on the other hand, comes in almost every color: vibrant pinks and greens, yellows and oranges, and the astonishing electric blue of Paraíba tourmaline. Within any gem's color range, some colors are more desirable than others. These top colors fall within limits that are generally accepted in the trade. A gem that features what the trade considers the most desirable color or colors is described as having fine color. Blue sapphire is an excellent example. Although blue sapphires range in color from violetish blue to strongly greenish blue, only a small portion of that range is considered fine color. Sapphires of violet-blue to pure blue are generally considered the best, and they are valued much more highly than

greenish blue sapphires. In some gem species, large, fine-color stones are much more plentiful than in others. In sizes above 2.00 cts., fine-color chrome tourmaline is much more available—and therefore a lot less costly—than tsavorite garnet of the same size and appearance. Most tsavorite rough is highly fractured and suitable only for small stones. This makes fine-color tsavorite larger than 2.00 cts. rare and expensive.