A Consumer’s Guide to Gem Grading


diamond cluster - gem grading
“Power & Resistance” by Alden Chadwick. Licensed under CC By 2.0.

The “Four Cs” of gem grading stand for color, clarity, cut, and carat (weight). Gemologists grade gemstones based on these properties. I’ll explain how we gemologists evaluate each of these properties and how consumers can use this information. However, first you must understand that gemstones are graded by the rarity of their features.

Even though we may use the term “quality” when describing the properties of gems, that doesn’t necessarily mean that one gem is better than another. Ultimately, you’ll purchase the gem that’s best for you personally. When a gem receives a high grade for a property, that means it has rare features within that property. What’s best for you may not necessarily be the “top” or rarest grade of the gem.

Feature Rarity and Gem Grading

Gemstone deposits will yield pieces with a wide variety of qualities. For instance, let’s say someone just mined 100 pounds of amethyst and brought them to a gemologist for grading. One of the first things the grader will notice is the size variation. The smaller stones will settle at the bottom of the pile. This leaves the larger stones on top, demanding the most attention. Since there are fewer large stones, they’ll receive a higher grade in the size category than the smaller ones.

For amethysts, a 100-carat stone is rarer than a one-carat stone. Therefore, that feature will add to an amethyst’s value. However, simply being larger doesn’t make it better. A small person couldn’t wear such a huge stone gracefully. It wouldn’t be the best gem choice for someone with a slight build.

The same principle applies to the other grading factors, too. As the grader continues going through the amethysts, only a few gems of the absolute deepest and richest coloring emerge. These will command special attention and pricing due to their rarity. Once again, however, consider what you want as a consumer. Bold colored gems aren’t the best matches for every complexion or personality. Softer pastels may suit some people much better.

amethyst color - gem grading
Amethysts, dark and light.

So, as you read about gem grading and Four Cs below, remember that grades reflect the rarity of the feature. They don’t measure beauty. With only one exception (cut), gem grades won’t indicate which gem better suits your needs.

When choosing a gem, use gem grades to help you look for a stone that will give you the most enjoyment.

Color

Gemologists describe gem color by referring to three properties: hue, tone, and saturation.

  • When most people describe colors, they most likely mean hue. The basic hues are red, orange, yellow, green, blue, violet, and purple. Black, white, and brown refer to tones and saturation, not hues.
  • A gem’s tone refers to a gem’s relative lightness or darkness. Black and white are tones, from darkest to lightest.
  • A gem’s saturation refers to the intensity of its hue. Colors can be strong or soft. Pink is desaturated red. Warm colors, like red and orange, become shades of brown as their saturation decreases.

Generally speaking, highest values go to stones with pure hues and strong, rich colors.

Color Grades for High-Value Colored Gems

With high-value gems, subtle variations make a significant difference in price. For example, a slightly orangish ruby won’t be worth nearly as much as a pure red. Most people can’t see the difference. To an expert grader, however, it makes a significant difference.

Orange-Red Ruby - gem grading
Orangish Red Ruby.

Color Grades for Moderate-Value Colored Gems

Color has less of an effect on the value of moderate-price gems. Tourmaline, for example, comes in every possible color. Unless a gem has an exceptionally pure green, red, or pink hue, color affects the value of all tourmalines equally. The other factors (clarity, cut, and carat) will determine the gem’s value.

Diamond Color Grades

Gemologists color grade diamonds on how close they come to colorless. The top three grades that indicate colorlessness for diamonds vary subtly only by differences in transparency. The next four grades cover “nearly colorless.” These grades indicate a gem will appear colorless when set in jewelry. It takes an expert in a laboratory setting to distinguish between these grades. However, each grade represents a significant change in value.

What Color Gems are Right for You?

The color that suits you best is a personal matter. Your personality, complexion, and even the clothes you want to coordinate with a gem will influence your choice. Look at a lot of gems. You’ll find that subtle color variations can significantly affect how they look on you as well as your emotional reaction.

For connoisseurs of fine-quality gems, higher grades are important. On the other hand, average consumers can find just as much beauty in modestly priced stones. Remember, quality doesn’t mean better, it means rarer.

Clarity

Gemstones contain a wide variety of inclusions. In faceted gems, inclusions are defined as anything that will interfere with the free passage of light. These can include little bits of minerals, hollow areas, and fractures. Clarity gem grading addresses the visual and structural impact of these things.

Diamond Clarity Grades

As with color, tiny differences only apparent to the grader can have a significant impact on value. Diamond clarity grading best exemplifies this. Several grades indicate diamonds with inclusions invisible to the naked eye, which have no affect on the stone’s beauty. However, the difference in value is substantial. At one end, you have something that an expert with 10X magnification can find with difficulty. At the other, you have something that is easy to find with magnification.

Colored Gem Clarity Grades

Gemologists simply grade most colored stones either “eye clean” (inclusions invisible to the naked eye), slightly, moderately, or heavily included. Eye visible inclusions always lower a gem’s value, but the change isn’t applied equally. Colored gems are also grouped into three clarity types: “Type I,” usually eye clean; “Type II,” usually included; and “Type III,” almost always included.

Emeralds are Special Cases

Emeralds fall into the Type III category. However, you can’t compare their clarity with any other gems. If you want an emerald without any eye visible inclusions, you’re limited to small stones. Otherwise, if you want a larger emerald, you must accept the inclusions and find value in the color.

emerald - gem grading
“Emerald-Cut Emerald,” Brazil. © Dan Stair Custom Gemstones. Used with permission.

If this compromise doesn’t appeal to you, consider other green stones, like tourmaline and diopside. Their color can equal that of fine emeralds, without the inclusions, and at a much lower price. Now, you must choose between having the name “emerald” or a premium green color.

Balancing Clarity and Price

Although eye visible inclusions always affect value, you can sometimes use this to your advantage. I remember one young woman showing off her diamond engagement ring. She had me look real close to see three tiny black dots. By accepting those small inclusions, invisible from further away than six inches, her fiancé could afford a much larger diamond.

You can also find just slightly visible inclusions in colored stones. Even some moderately included gems, like a dark garnet, will look fine when viewed from a normal jewelry distance. You’ll have to decide if this will interfere with the joy you get from the stone.

garnet - gem grading
“Mixed Scissor/Step Triangle Cut Spessartite Garnet,” Nigeria. © Dan Stair Custom Gemstones. Used with permission.

Inclusions and Structural Integrity

While beauty lies in the eye of the beholder, fractures and veils can lie deeper in the gem. Pay special attention to these inclusions. They represent a weakness in the gem, which may be prone to breakage. Earrings, pendants, and brooches don’t receive much abuse. Ring stones, however, take a constant barrage of bumps and bangs. If you’re looking for a ring stone, avoid a weak gem.

Non-professionals will find identifying these inclusions difficult, let alone determining their impact on structural integrity. The old saying, “If you don’t know your gems, know your gemologist,” applies here.

Cut

A gemstone’s cut, the workmanship that went into fashioning it, is one of the most important factors in gem grading. It has a tremendous effect on a gem’s appearance. The cut is also one of the most difficult qualities for the non-professional to judge, due to the variations involved. Nevertheless, consumers can conduct some basic examinations.

Symmetry

First, look at the shape of the stone. Of course, some gem cutters carve gems into beautiful “freeform” shapes. Most gems, however, are intended to have regular shapes. If this is what you’re looking for, look at the symmetry. Does it bulge here or there, or is it symmetrical in all directions? Look at the stone from the side and the ends. Again, it should be symmetrical in all directions. If not, you must decide how much that will affect your appreciation of the stone.

Asymmetrical Cut - gem grading
Asymmetrically cut gemstones.

Magnification

Insist on inspecting any gem you’re considering buying under magnification. Look at the surface areas that reflect light. They should be smooth and mirror-like. Pitting, scratches, or dull areas indicate a poor polish. Such a gem may look good in the store, but someday you’ll compare it with a well-polished gem. You’ll be disappointed with your purchase.

polish comparison - gem grading
Compare a poorly polished rhodolite garnet (left) with a well-polished one (right). © Dan Stair Custom Gemstones. Used with permission.

If you’re examining a faceted gem, look at the facet junctions, too. On a well-cut stone, they’ll be crisp and come together at a single point. You may see slightly rounded facets not quite meeting where they should. Just how far off will affect the gem’s brilliance.

Compare Similar Stones

While other cut factors may be too complex for non-professionals, you can get a good idea of a gem’s cutting quality simply by comparing it with other gems. When you do this, make sure to compare similar stones. For example, an amethyst will never have the brilliance of a topaz. Dark stones don’t have the brightness of light colored ones, etc.

When comparing like stones, judge the overall light return. Let’s say you examine two like gems of the same size and color. If you find one has much more brilliance and sparkle, you’re seeing the impact of cutting.

Brilliance comparison - gem grading
Compare the brilliance of these two tsavorite garnets.

Windowing

Pay particular attention to windowing, a condition where light passes straight through the gem’s center rather than reflecting back. You can spot this easily. The center will be much lighter than the outside of the gem and have no flashes of light. Some windows are small. Others are quite large and hideous.

Like a poorly polished gem, a gem with a large window might look good in the store. Eventually, however, you’ll notice the difference between a well-cut stone and what you bought.

windowing - gem grading
Large window (top) vs. small window (bottom).

Cabochons

Non-professionals have an easier time examining cabochons or cabs, gems cut with flat bottoms and domed tops.

Start by checking the polish under magnification. Then, hold the stone a short distance from your head and rotate the gem slowly. Notice how the light passes across the surface. On a well-cut cab, it will flow smoothly from one side to the other. If poorly shaped, the light won’t flow smoothly but will snake across the surface. You can also find surface irregularities and poorly polished areas this way.

Carat

A carat is a unit for measuring gem weight. One carat equals 1/5 of a gram, or 200 milligrams. One kilogram (approximately 2.2 pounds) equals 5,000 carats.

Increases in Price per Carat

Simply put, larger stones are less common than smaller ones. Hence, they often demand a higher price per carat. For example, a quarter-carat topaz may cost $60 per carat, or $15 total. A half-carat topaz, with the same color, clarity, and cutting grades, might cost $100 per carat, or $50 total. For a topaz one-carat and over in size, the price could reach $200 per carat. So, a one-carat stone will cost $200 total, not $60 or $100.

Choosing Large or Small Gems

Choosing the right gem size is a personal matter. Bold personalities might prefer large gemstones. Someone with delicate sensibilities might prefer smaller gems. Most people will likely fall between these extremes.

For tighter budgets, smaller stones have a significant advantage. Not only do they cost less per weight, the “amount of gem” you see is disproportionate to size. This is because gem volume goes up faster than outside dimensions. For example, a ½-carat, round diamond measures 5 mm in diameter. A ¾-carat diamond measures 6 mm. A full carat diamond measures 6.5 mm.

To a casual observer, the ½ and ¾ carat stones seem about the same size. The ¾ and full carat stones also look about the same size. However, their price differences can be considerable.

Clusters

Jewelers often cluster small gems to give the illusion of more gemstone.

Seven 1.6 mm diamonds, set close together, will take up as much space as a whole carat diamond. If set on white gold, distinguishing the separate stones proves difficult. Hence, these settings are often called “illusion settings.” While these seven stones approach the visual appeal of a one-carat diamond, they only weigh 0.14 carats. Considering that the price per carat for small diamonds is also much lower, you’ll see a significant cost difference. A cluster ring’s price could range in the hundreds of dollars, rather than the thousands.

You’ll find colored gems often clustered for the same reason. You’ll get more “visible gem” for less money. If your budget can’t accommodate your first gem choice, consider a setting with several smaller gems. You might just find the look and emotional appeal you wanted at a price you can afford.

Conclusion

Now you know the factors that go into gem grading. Remember that rarer features don’t necessarily mean the stone is “better quality.” Stones smaller or lighter than the top grades may be just right for you.

However, keep in mind that clarity can have an impact on a gem’s durability and jewelry use. Also, cutting can greatly affect a gem’s appearance, so examine cut gems carefully. Always choose the best cut you can afford. (No personality will find a poorly cut stone appealing).

Nature doesn’t always provide perfect gems, nor do lapidaries always produce perfect work.

Be ready to make compromises. You may not be able to afford your first choice of gems.

When shopping for a gem, find what suits your personality best, within your budget.

Finally, the most import factor to consider when gem buying is the joy you’ll get from your gem.