The 4 C’s of Gemstone Valuation

The valuation of a gemstone is derived from the “4 C’s”: carat, color, clarity, and cut. Understanding all four of these is vital background to the buyer. If you are buying an expensive stone with a price in four figures or more, buy it loose so you can see it weighed and be able to fully examine it.

Of course, this will not always be possible; some jewelry is already set, for example, if you were buying a piece of jewelry from an estate or from an auction. Some gemstones are mounted in a bezel or other setting that would be damaged upon removal.

Glyptics by Bart Curren show contemporary gemstone carvings
Photo by ICA/Bart Curren

Carat Weight

Carat is a weight that is equivalent to one-fifth of a gram. Originally gems were weighed in hand held balance scales with two pans. The gems were in one pan and carob beans were used on the other side. The word carat was derived from “carob”. Today we use electronic digital scales to weigh gemstones.

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Carat weights should be given on a gemstone to the one-hundredth of a carat, for example l.46 carats or 8.65 carats. If one is buying a piece of jewelry with multiple stones, total carat weight also should be stated to the nearest one-hundredth of a carat. In the past carat weights were sometimes expressed in fractions. A gemstone might have been sold as 5/8 carat which should mean exactly 0.625 carat. But often this really meant that the weight was somewhere between 0.60 and 0.65 carat. Today this inaccuracy is unacceptable. Because gemstones are priced by the carat, carat weight must be specified to the one-hundredth of a carat.

One hundredth of a carat is referred to as a point. There are one hundred points in a carat. Jewelry sold through some catalogues does not always have carat weight listed; this poses serious problems to the buyer; since you would have no idea of carat weights.

In general, a gem cutter will try to maximize the yield when cutting a gemstone from a piece of rough. The size and shape of the rough usually determines what the shape of the gem will be and what the resultant carat weight will be. The exception to this would be when the rough material comes in very large sizes and the cost of the rough is inexpensive; then the cutter will become more creative with the rough and cut a variety of shapes and sizes. Amethyst is a good example of rough available in large sizes at a relatively inexpensive cost especially in light colored material. On the other hand, fine quality ruby rough that can cut stones above a carat is extremely rare, and therefore expensive, fine rubies are almost always cut in shapes that “fit” the crystal and save the most carat weight. Usually this means you will see rubies in oval or cushion cuts.

Each gemstone mineral has a different density, so different gemstones of same millimeter size will have a different carat weight. A well cut round brilliant cut diamond weighing exactly one carat will have a diameter of about 6.5 mm.; an amethyst with a 6.5 mm. diameter will weigh about .80 carat; a natural zircon with the same diameter and proportions would weigh about 1.40 carats.

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Not everything is possible when one is looking at nature’s achievements. You should think about nature’s rules before you shop. It will save you a lot of frustration and money if you eliminate very difficult to find sizes and shapes.

Color

Color is the most critical factor affecting the price of any gem variety at a given size. Color can be separated into three basic components: hue, saturation, and tone. Hue is what we usually think of as color. It is described by the name of the color of the stone (i.e., red, orange, yellow, green, blue, and violet.) Some hues are in-between two colors which are next to each other in the spectrum, this can be described by combining hues as in blue-green, red-violet, blue-violet, and so on.

Saturation (or intensity) describes how much color there is. The highest valued gemstones in each variety are those with the most color saturation; the brightest color! Price falls as saturation is lowered. Emerald prices are very dependent on saturation, the brighter the green the higher the price; prices drop quickly as color approaches a “mossy” color of green.

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Tone (or value) describes the lightness or darkness of a color. The term tint can be used to describe lighter values and shade for darker values. Each gem variety has an optimum tone. Stones that are too dark or too light will drop in price as it varies from the “ideal” tone. The best amethyst have a medium dark violet color; price drops as the stone approaches a pastel violet color or a very dark violet.

Generally, the more pure the color and the more saturated it is, the higher the price. For example, rubies that are most valuable are as red as possible without overtones of violet or orange. It should not be too dark or too light. The same is true for emeralds: the best color is considered to be intense green without an overtone of yellow or blue but not too light and not too dark.

Not only are gems of top color the most costly, they also may not be easy to find, especially in ruby, sapphire, emerald, Imperial topaz and aquamarine. If you have a budget to consider, should you buy a less expensive color? Some of the gems with hints of secondary colors may be extremely attractive. Some people prefer pastel or muted tones to primary bright tones. Don’t be afraid to pick the color you like: gems should be bought because their beauty strikes the buyers taste. If you like a light blue sapphire or an extremely dark one, buy the one that will make you happy when you look at it.

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Topaz, 16.09 carats, Brazil.
Photo by ICA/Bart Curren

As a buyer you must be very aware of the conditions under which you are buying a gemstone. First, note the background color upon which you are looking the stones. White is the best possible background color for observing any gem material. If you are looking at diamonds or pearls they may be presented on a black background; ask to view these items on a white background. White, as a background, allows you to see the true color of the gem material. Other background colors will distort the apparent color of what you are looking at.

Second, observe the light source used for viewing the stone. If you are in a jewelry store the light may be from a spot light or some other very intense light source. When the stones are viewed under this kind of intense illumination you see the utmost potential of the stone’s brilliance and scintillation. Wow, it’s dazzling!

Now let’s change light sources and look at the stone in a more realistic way. Under incandescent light, regular fluorescent light and shaded daylight you will see how the gemstones looks under normal conditions. If you have a penlight keep it in your pocket when you are looking at the color and overall brilliance of the stone. A penlight will have the same enhancing affect as the spotlight. Be aware that the color of a gem may appear very different under different light sources. Different light sources will have different color components; they will often affect the apparent color of the gemstone. If a stone looks good in soft light, it will look spectacular under strong light. Don’t be afraid to ask the seller to show the stone in less than optimum lighting conditions. Look at the stone in shaded daylight, not in direct sunlight.

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Keep in mind that some gemstones appear different colors under different light sources. The ones that will be most notable are tanzanite, violet sapphires, and sometimes pink tourmaline. Natural alexandrite of fine quality will show dramatic color change under different light sources.

Some gemstones are noticeably color zoned, i.e. color is unevenly concentrated in one or more sections of the stone. Blue sapphires and amethyst are the most common example of stones that may be color zoned. In a sapphire there may be a blue spot of color in an otherwise nearly colorless crystal. Sri Lankan cutters can expertly cut these so that when the stone is face up it appears to be a magnificent blue. In the trade the term faces up well means the stone looks perfectly acceptable when it is viewed as it would in a mounting, but something about it is less than perfect. For jewelry purposes, stones that look great in a mounting may be perfectly adequate as long as you pay less than you would if all aspects of the stone are top notch.

Color concentrations may be obvious in concentric bands in sapphires and in amethyst. If this detracts from the gemstone’s overall beauty, the price should be much less. Sometimes in an amethyst, color variations may occur in large sections. An amethyst may be a blue violet in one area and a red violet in another; this may impart a special richness to the stone. Stones have individual variations; each must be judged on its own particular merits, but the most important consideration is the stone’s beauty. What, in one gemstone, would be an imperfection in another gemstone may create its own distinctive beauty.

Clarity

Clarity refers to the internal landscape of inclusions that is inside the gem or blemishes that are on the exterior of the stone. The inclusions may be minute crystals of the same mineral, crystals of another mineral, tiny gas bubbles, small liquid filled pockets, internal fractures or cleavages, or any other visible matter inside the stone. These may form as one or more crystals or form in fine microscopic “clouds”.

Internal fractures are common in some varieties of gems; if they are large or extensive, avoid buying the gem because the durability may be affected.

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Blemishes are external features that include chips, scratches, abrasions, naturals on diamonds and spots on pearls.

The inclusions in a diamond greatly influence the value of a diamond; in colored gemstones the inclusions generally have less bearing on the value. Some gemstones have eye visible flaws that will detract from the gem; others have microscopic inclusions that will not detract from the stone’s beauty. Inclusions in colored gemstones should be considered a negative factor if the durability could be affected or if the brilliance of the gemstone is lessened. Inclusions are not always a negative feature. Some inclusions can enhance the beauty or add to the mystery of a gemstone; it is the inclusions that create stars in asterated gemstones and the flash in moonstone. Inclusions can identify a gem or indicate its origin, even add to its value. In diamonds each flaw or blemish lowers its clarity grade, hence lowers its value. In rubies and sapphires, inclusions may pinpoint where the stone was mined or whether it was heated.

In accordance with FTC (Federal Trade Commission) guidelines, diamonds should be clarity graded by a ten-power hand held loupe or microscope. The way a diamond reflects light within the stone makes it difficult to see inclusions with the naked eye. Clarity grading of diamonds will be thoroughly discussed in the chapter on Diamonds.

Colored gemstones do not have rigid guidelines for grading clarity. Colored gemstones can be viewed in a couple of ways to quickly assess clarity without magnification. First, put the stone table down on a white surface or piece of paper. Any substantial inclusion will be clearly visible. The second method requires a penlight with a flat area where the light comes out; merely place the stone table down over the light source; when the light is on, most inclusions will become very obvious.

Cut

Cut refers to the shape of the gem, the proportions of the various parts of the gem, the finishing touches such as facet relationships, and, finally, the polish. Cut is critical in creating the full potential of beauty in a gemstone. It is most critical in diamonds, where color is a less obvious consideration.

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The shapes available today are quite varied: rounds, ovals, cushions, pears, emerald or octagonal cuts, hearts, marquise, hexagons, half moons, triangles, and carved gems are a few of the possibilities.

The basic parts of a traditional faceted gemstone are the crown, girdle, and pavilion. The crown denotes the top portion of the stone, the pavilion is the lower portion; the girdle divides the crown and pavilion. The table is the top facet of a gemstone and usually the largest. Some gems have facets where the table normally is; these are called table cuts.

Each transparent gem material has a critical angle that is a function of the refractive index. To achieve maximum brilliance from a gem the slope of crown and pavilion angles must be cut to those critical degrees. A diamond that has proper critical angles and good proportions will trap all the light that enters the gem and reflect it back out as white light. This is called brilliance. A poorly proportioned diamond leaks out light from the bottom, not all is reflected back up. Optimum brilliance is not achieved.

In colored gemstones there are a number of variables that affect cut; l00% brilliance may not be the ultimate goal in cutting a colored stone. For example if a gem material is too dark, the stone may be cut shallow; this gives the stone a better looking color, but brilliance is sacrificed. In very dark stones brilliance is not always apparent because the light is absorbed rather than being reflected out as brilliance. Very light colored gemstone rough material might be cut very deep so that the overall color will appear darker. Or the cutter may just cut the gem for maximum yield with all other factors disregarded. You must look at the gemstone and judge why it was cut in a particular manner.

A well cut gemstone reflects out l00% possible brilliance. One that is too shallow will have a dead spot in the center of the gem where light is lost out through the back. A deep stone will absorb too much light without reflecting back the brilliance; it will also be heavily bellied and may be difficult to set. You will pay for extra carats which don’t add to the gemstone’s beauty.

Asymmetrical pavilion is easily visible in this yellow sapphire. Photo by ICA/Bart Curren

Paradoxically, very expensive gem materials may have the least desirable proportions. This occurs when the rough material is exceptionally good in color and the rough is a shape that does not lend itself to be cut into good proportions. Rubies, beryl, sapphires, and alexandrite chrysoberyl may not always have optimum proportions for these reasons. The cutter generally cuts this kind of material for maximum yield. Less expensive rough such as blue topaz, amethyst, and citrine will often have excellent cuts so they will be more beautiful hence more salable.

The shape of the rough will often determine the finished shape of the gemstone in moderate and expensive gemstones. Common examples of these are emerald cuts in emerald and in green tourmaline . Cutting in these shapes minimize weight loss from the rough.

A stone that is rounded into a dome on top and not faceted, is called a cabochon; the word is derived from an archaic French word for “head of cabbage.” Stones that are cut into cabochons are generally more included than faceted stones. The cabochon cut is ideal for stones that are translucent or opaque rather than transparent. It displays the asterism of star stones and the chatoyancy of eye stones.

Stones may be cut with cabochon tops and faceted bottoms; these are called buff tops. Today we see many carved gemstones; they may be cabochon or faceted with carving. Many stones are appropriately called designer stones. Some cutters are truly artists who create their own distinctive line of gemstones cut in their own individual fashion. Some of these are signed. You will pay more for an exceptionally well cut stone, a distinctively cut stone, or a signed stone.

Finishing Touches

The finish of a gem is important in maximizing the beauty. Facets should be regular in shape and in alignment with each other. This creates scintillation in the gem. Scintillation is the reflection of light off a facet surface. It is like the twinkling of the stars. The final polish is critical to the surface luster of the gemstone and it is important to the scintillation of the gemstone. Very low cost, faceted gemstones rarely have a good polish, as these are worn and they become dirty, they appear inordinately dull. Well polished gems have a noticeably better surface luster.

Where Gemstones are Cut

Most of the world’s supply of gemstones are mined and cut in underdeveloped countries where labor is cheap.

Brazil is a major source of tourmaline, topaz, aquamarine, emerald, amethyst, citrine, peridot, beryls, andalusite and more; gems cut in Brazil may be well cut but the majority of low cost gems are cut in a mediocre fashion.

African nations are sources of diamonds, tourmaline, emerald, topaz, garnets, amethyst, tanzanite, and much more; but most of this rough is exported and cut elsewhere. Sri Lanka, Thailand, India, and Burma are sources of most of the world’s supply of fine rubies and sapphires; most are cut in the country of origin. A great many of the gems cut in Burma or Sri Lanka are re-cut in Bangkok, Thailand! Australia mines great quantities of sapphire and opal; most is exported for cutting. Thailand currently is the world’s major cutting center. Thai dealers search the world for gem rough to cut in Thailand. Thai cutting ranges from mediocre to very fine. German gem cutting goes back many centuries; German cutting is usually good to exceptional. Hong Kong imports gem rough and has a small industry of fine cutters and carvers.

Diamonds are mined in African countries, Russia and in Australia. Most diamond cutting is done in India, Israel, Belgium and in USSR. Fine large diamonds are cut in the United States. Diamond cutting is a growing industry in Thailand, Sri Lanka, and the People’s Republic of China. Contrary to common thought, Amsterdam is not a major diamond cutting center; it is an excellent tourist attraction but not important in the wholesale market.

Where a gemstone is cut generally determines the quality of cutting and can greatly influence the price of the stone.

Country of Origin

Country of origin could be the fifth “C”. In diamonds this is not a consideration. Top quality stones that command a premium price if the origin can be authenticated include: Colombian emeralds, Burmese rubies, and Kashmir or Burmese sapphires . If one is in Montana, sapphires that are locally mined will demand a higher price there than a stone of identical quality would if sold elsewhere. The same is often true for opals in Australia. This is common for many local gem products in this country. We could call this the souvenir effect. In most gems the color and clarity will be the primary determinant of value regardless of origin.

See also:

Gem Dealers’s Secrets – Handbook for the Gem Buyer – Table of Content

By Sondra Francis - Copyright © Sondra Francis, G.G. 1999
About the Author: Sondra Francis has scoured every major colored gemstone market in the world since 1978. She was a charter member of the American Gemstone Association and served as a board member. She was a founding member of the International Colored Gemstone Association. A true gem lover, Sondra has marketed her treasures on the wholesale and retail markets.
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Acknowledgments: A special thanks to Pam Dulgar, Alex Edwards, Cheryl Kremkow, Kate Kirby, Helen Mitchell, Carol Morgan Page, David Pond, Elaine Proffitt, and Ray Zajicek for their help.
Photographs: Bart Curren and ICA Gembureau ; Alex Edwards, Pearl Sales Institute ; David Dikinis
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