Techniques for Grading Colored Gemstones

Scientists estimate that the human eye can distinguish some 1 million separate colors. That’s just where the trouble begins in grading colored gemstones.

Most jewelers will never stock all of the roughly 100 colored gemstones available today, but they may keep 25 or so on hand at any one time. Grading these stones is similar to grading diamonds in that three of the “four Cs” apply. Yet the rich complexity of colored gems makes the task considerably more challenging.

Of the four Cs – color, clarity, cut, and carat weight – the first three pertain to a gemstone’s overall beauty, rarity, and desirability. Carat weight factors into a gem’s price but not its grade.

Color. This is by far the most important component of a colored gemstone. Some dealers believe color accounts for no less than half the gem’s value. Clarity and cut are still important, but the first impression always comes down to color.

Describing color is a tricky game in our industry. Sky blue, grass green, bubble-gum pink – these are just a few of the many creative terms jewelers use today. While these descriptions may conjure an impression of a color, they can’t fully convey the subtle nuances at play.

Several color-grading systems are in use today. (Some provide helpful information relating to the 1-to-10 grading scheme used in my publication, The Guide.) Every jeweler, gemstone dealer, and appraiser should own a color grading communication tool. (See p. 109.) Most do not. The excuses include high cost, difficulty of use and transportation, and lack of a single jewelry industry standard.

There are three components to color: hue, tone, and saturation. Hue refers to the pure spectral color, describing the dominant and any additional colors visible in a gem. Examples include blue, greenish blue, red, orangy red, and slightly purplish red. The Gemological Institute of America’s color grading course refers to 31 hues.

Tone is the lightness or darkness of a color sensation. In the GIA color-grading system, tones range from very light to very dark, with five intermediate levels. Saturation (sometimes called intensity) is the strength or purity of the hue. The GIA system includes six levels, starting with a brownish or grayish saturation and progressing up to vivid.

Each gem variety has an optimal hue, tone, and saturation. Dealers will argue about which combination is best. But most would agree, for example, that the ultimate ruby is a pure red (without secondary orange or purple), with a medium-dark tone and vivid saturation.

An aquamarine, on the other hand, calls for a different optimal color mix. The desired hue is blue (or, as some prefer, slightly greenish-blue); the tone should be less dark than a ruby, approaching medium light to medium; and the saturation will probably not exceed medium (level 4 in the GIA system).

Some dealers develop their own master stone sets for expensive gems such as rubies. While the cost of doing so is prohibitive to all but a few in the trade, these dealers recognize that slight color nuances in a major gem can put thousands of dollars at stake. Photos of color such as the ones that appear in this article are helpful benchmarks for quality, but they can never convey the true color and depth of an actual gemstone.

There’s one more factor to consider when you grade color: light. When possible, you should view gems in north daylight. If that’s not feasible, use the best daylight-equivalent lighting you can buy. Artificial lighting can alter the way a gem appears. For example, incandescent lights will make a ruby appear more red and hide the secondary purple color.

Clarity. The clarity of a colored gemstone is usually the second most important criterion for grading. Check the clarity first with the unaided eye and then with 10x dark-field illumination. Nature does not grow each gem variety on a level playing field, so take into account the gem variety when judging clarity.

GIA developed a classification system to differentiate among the various gem species and how they grow in nature. Type I includes gems that are typically very clean internally. A Type I gem graded SI without eye-visible inclusions would have a lower overall quality grade. The overall quality of a gem with eye-visible inclusions would be lowered even further.

Type II are gems that grow moderately included. For most of these, the actual clarity grade will have little influence on price, unless the gem has eye-visible inclusions. In that case, the clarity will downgrade the overall quality and thus lower the value of the gem.

Ignoring the flawless and VVS grades – which in any case are extremely rare for a Type II gem – there’s little price differential for a VS grade vs. an SI grade. As the inclusions become more prominent to the unaided eye, the effect will be more severe.

Type III gems are usually highly included. These are graded more leniently than the others. Since emeralds by nature are very included, slightly eye-visible inclusions are the norm. In that case there would be little, if any, deduction in price. Excessively included emeralds would require greater deductions. Gems without eye-visible inclusions may command a premium, although color is still the dominant price factor.

Different gemological laboratories use other clarity-grading methods. For example, the American Gemological Laboratory in New York uses FI (free of inclusions), LI1 and LI2 (lightly included), MI1 and MI2 (moderately included), HI1 and HI2 (highly included), and EI1, EI2, and EI3 (excessively included).

Cut. The cut is generally considered the least critical factor in grading a colored stone. But it can still play an important role.

Often, gems such as rubies are seen in “native” cuts with excessive bulge, deep pavilions, and shallow crowns. While the intent here is clearly weight retention, the gems may still command high value.

A poorly cut gemstone is likely to exhibit windowing or extinction. Windowing is the read-through effect caused by leakage of light. Extinction is a darkening of certain areas of the gem. While the deduction for a poor cut may be small, windowing or extinction can adversely affect the color grade.

By contrast, if a gem is very well cut, it may appear much more brilliant, and the cut may improve the color appearance and enhance the color grade.

You need to consider many proportion and finish factors in grading the cut. Among the proportion factors:

  • Outline balance. In the face-up position, the gem should be equally balanced and well-shaped.

  • Length-to-width ratio. The guidelines for this factor are listed here. But remember that aesthetic appeal plays an important role in assessing length-to-width ratios. Even if these parameters are not met, the cut may still be acceptable if the shape is pleasing to the eye.
    Cushion: 1.50:1 to 1.75:1
    Rectangular: 1.25:1 to 2.00:1
    Oval: 1.33:1 to 1.75:1
    Pear: 1.50:1 to 1.75:1
    Marquise: 1.66:1 to 2.50:1
    Heart: 1.00:1 to 1.25:1

  • Profile balance. When the gem is viewed from the side, the table and culet should be centered, the girdle should not be wavy, and the pavilion bulge should be even.

  • Total depth percentage. For colored stones with acceptable crown and pavilion angles, the total depth percentage should normally be between 60% and 70%. Deviations from this range may be acceptable, depending on the overall eye appeal. Some shapes may dictate shallower or deeper cuts.

  • Crown height and pavilion depth. If the total depth percentage is between 60% and 65%, then 1/4 to 1/3 of the stone should be above the girdle and 2/3 to 3/4 below.

  • Bulge. Most colored stones will show some bulge in the pavilion. Minor bulge is acceptable, but obvious bulge is not.

  • Table size. Table size varies more with colored stones than with diamonds, and may range from 30% to 80%. A table that appears obviously large to the eye detracts from the cut grade.

  • Brilliance. The light returned to the eye is known as brilliance. A well-cut gem will have maximum brilliance with minimal windowing (light leakage) and extinction (dark areas).

The finish criteria include:

  • Polish. This concerns the surface characteristics of the gem.

  • Symmetry. Here, you consider the shape, position, and arrangement of the facets.

Designer-cut gemstones warrant special consideration. Simply raising the grade of the gem to account for the designer cut is not acceptable. Grading them all as a “10” is likewise insufficient; this would equate these cuts to a well-executed standard cut that might also rate a “10.”

For designer-cut gems, grade the material as is for color and clarity only, and consider the designer cut as a separate issue. The added time involved and the provenance of the designer will add value. Usually, greater weight loss will occur, and this too must be taken into consideration.

Putting it all together. Start with a worksheet for grading. Assess the color grade using a color-grading system or by visual analysis. For example, if the color of a ruby is slightly purplish-red with a medium-dark tone and strong saturation, the initial color grade might be in the lower-fine category, perhaps grade 6 or 7. Try to visualize this gem. The slight purple secondary color disqualifies it from the extra-fine category, but the tone and saturation are desirable for a ruby.

Now consider the clarity. Ruby is a Type II gem, so some inclusions are expected. If the gem doesn’t have eye-visible flaws and 10x observation reveals only moderate inclusions, the grade will remain unaffected. Exceptional clarity may boost the grade by1/2. On the other hand, heavy inclusions under 10x may reduce it by1/2. Prominent eye-visible inclusions will have a much greater effect of one to three grades lower.

The last consideration is the cut. Assuming a standard cut without designer premiums, consider all the factors described above. The typical average native cut is expected for a ruby. If it’s a better-than-average cut, add about a1/2grade. If the bulge is a bit excessive or other facet characteristics are slightly off, you may reduce the grade by a1/2. If the cut is poor, deductions of one to two grades may be necessary. Remember that a poor cut can also compromise the color grade.

Consistency is key. With so many variables to consider, jewelers, dealers, and appraisers naturally differ in how they grade gems. Even trade laboratories vary in their approaches to colored gems. The GIA Gem Trade Laboratory, for example, has never included color-grading notations on its colored gemstone reports. The lab simply identifies gems as natural or synthetic and provides enhancement information. Other gem labs, for example the American Gem Laboratory in New York, include grading information.

In the absence of universal grading parameters, find a method that works for you and stick with it. A consistent approach makes grading color more enjoyable as well as profitable. In any event, don’t fear colored gems. They may be more complicated to grade, but they’re also more fun. And who doesn’t enjoy a good challenge?

Richard B. Drucker is the president of Gemworld International and publisher of The Guide, a pricing publication he began in 1982. An international gemstone consultant, he has published numerous books in the jewelry industry.

GIA Type Classifications for Major Gemstones

Aquamarine Andalusite Emerald
Chrysoberyl – yellow/green Alexandrite Red beryl
Citrine Sapphire – all varieties Tourmaline – red (rubellite)
Green beryl Ruby
Kunzite Garnet – all varieties
Morganite Iolite
Smoky quartz Peridot
Tanzanite Amethyst
Topaz – all varieties Spinel – all varieties
Tourmaline – chrome, green Tourmaline – bi-color, indicolite, golden, pink
Zircon – blue Zircon – all varieties except blue

Calling ’Em as You See ’Em

There is no universally accepted color grading system. Instead, graders use various terms to describe the grade of a gem. Some grading systems incorporate the traditional A, B, C grading philosophy. In theory, this could work. The problem is that some gem suppliers complicate matters by using AA, AAA, and even AAAA grades. Some stop at C, while others extend the alphabet down to F.

Some catalogs break from tradition altogether by grading on a unique numbering system. These systems work nicely for users who can understand them. Others may find themselves at a loss.

My publication, The Guide, divides color grades into four categories: commercial, good, fine, and extra fine. A number from 1 to 10 is then assigned to these grades: 1-4 = commercial, 4-6 = good, 6-8 = fine, 8-10 = extra fine. Of course, no one is compelled to use these terms or grades. But the words and numbers clearly convey a sense of the gem’s overall quality.

Color Grading Communication Tools

Explore the options of each of these color grading systems and choose the one that best suits your needs. Although each has its limitations, you’re better off using one of them than none at all.

  • The Gemological Institute of America’s GemSet contains 324 plastic-molded “gemstones” in various hues, tones, and saturation levels. Although the coverage of colors is limited, the set is a handy retail sales tool, enabling the customer to relate to the gem samples.

GIA originally offered grids that correlated the GemSet samples to a grading system of 1 to 10. Although the set no longer includes grading information, grading grids in The Guide correlate GemSet to a grading system of 1 to 10. The set sells for $635.

  • The GemDialogue color communication tool from GemDialogue Systems Inc. of Rego Park, N.Y., is a set of 21 transparent color scale charts painted with inks. Each chart has 10 “zones” representing saturation for a total of 210 samples. Also included are black and brown overlays. By combining the samples and overlays, you can obtain some 60,000 color combinations. The set also comes with a grading book that rates the colors for each gem variety from 1 to 10. GemDialogue sells for $350.

  • The World of Color, distributed by the Gem Quality Institute’s Los Angeles and Chicago laboratories, is a pocket-size book based on the Munsell color spacing. Instead of the full 1,566 opaque chips found in the Munsell Book of Color, this edition contains charts for 31 standard colors and uses small color dots representing tone and saturation. A new edition is in production; the price has not yet been determined.

  • Color/Scan is a system developed and used by the American Gemological Laboratories in New York. It consists of a series of cards, each with six foil-like acetate samples simulating colored gems. The cards represent 156 color samples. They’re designated by gem variety, so that ruby, sapphire, and emerald, for example, each have separate cards. This set is not commercially available and is used exclusively in AGL’s lab for its grading reports.

Some companies have other grading sets that customers can use when ordering. One is the ColorGrid from Spectral Gems of Birmingham, Mich., a one-page laminated color chart that sells for $25. Although the chart doesn’t provide color grading information, it’s a helpful color communication tool nonetheless. Using the chart to better describe the color, Spectral Gems can help with the grading and pricing process.