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.
Gem Grading and Quality
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.
Gemstone deposits will yield pieces with a wide variety of qualities. Let's say, for example, 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.
Feature Rarity and Gem Grading
The same principle applies to the other grading factors, too. As the grader continues going through the amethysts, only a few gemsof 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.
"Siberian" amethysts have deep, rich purple colors with flashes of red and blue. Traditionally, amethysts with this rare coloration have received the highest values. However, "Rose de France" amethysts, with light, pinkish violet colors have become very popular gems. "Siberian" amethyst, 25.81 cts. "Rose de France" amethyst, 3.70 cts. Photos courtesy of liveauctioneers.com and Jasper52.
So, as you read about gem grading and the 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 grading to help you look for a stonethat will give you the most enjoyment.
Color
Gemologists usually describe gem color by referring to three properties: hue, tone, and saturation.
- When most people describe colors, they probably 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.
For colored gemstones, 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 gem grading and 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.
Color Grades for Moderate-Value Colored Gems
Color has less of an effect on the gem grading and value of moderately priced 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 highest diamond color grade is colorless. (Note that this differs from the highest color grades for colored gemstones).
The top three grades that indicate colorlessness for diamonds (D to F) vary subtly only by differences in transparency. The next four grades cover "nearly colorless" (G to J), which means diamonds with these grades will appear colorless when set in jewelry. It takes an expert in a laboratory setting to distinguish between grades G to J. Nevertheless, each grade represents a significant change in value. (Try judging diamond color from face-up photos for yourself in thisquiz).
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 grading addresses the visual and structural impact of these things.
When viewed from a certain angle, you can see an eye-catching series of dissolved, needle-like inclusions in this 8.70-ct, rectangular step-cut apatite. Otherwise, the gem appears eye-clean. © The Gem Trader. Used with permission.
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
When gem grading most colored stones, gemologists simply note whether a gem is"eye-clean" (with inclusions invisible to the naked eye) or else slightly, moderately, or heavily included. Eye-visible inclusions always lower a gem's value, but the change isn't applied equally. In the GIA clarity grading system, 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 that of 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.
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 gem grading and 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.
Just slightly visible inclusions also occur in colored stones. Even some moderately included gems, like a dark garnet, will look fine when viewed from a normal jewelry distance. Only you can decide if this will interfere with the joy you get from the stone.
Inclusions and Structural Integrity
Beauty may be skin deep, butfractures and veils go deeper into 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 conductsome 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 or is it symmetrical in all directions? Look at the stone from the side and ends. Again, it should be symmetrical in all directions. If not, you must decide how much that will affect your appreciation of the stone.
Magnification
Insist on inspecting any gem you're considering buying under magnification. Look at the facets, the surface areas that reflect light. They should be smooth and mirror-like. Pits, 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.
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 ones, etc.
When comparing similar stones, judge the overall light return. Let's say you examine two similar gems of the same size and color. If you find one has much more brilliance and sparkle, you're seeing the impact of cutting.
Windowing
Pay particular attention to windowing, a condition where light passes straight through the gem's center rather than reflecting back to the viewer.
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.
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 moves across the surface. On a well-cut cab, it will flow smoothly from one side to the other. If poorly shaped, the light will instead "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 exterior 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.
Toa 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, they can have considerable price differences.
Clusters
Jewelers often cluster small gems to give the appearance 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, remember 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).
Keep these four things in mind when shopping for gems:
- 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.
- Find the gem that 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.
If you have a particular gem type in mind, take a look at our gem buying guides. You can learn more about the specific factors that go into grading that gemstone as well as the range of options consumers may have.
