Choosing, Rating, and Grading DiamondsChoosing, Rating, and Grading Diamonds

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Choosing, Rating, and Grading Diamonds

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The most popular gems in jewelry today are diamonds. Their great brilliance and dispersion (the ability to take white light and display flashes of color or "fire" to the viewer) attract much attention. However, when it comes to choosing, rating, and grading diamonds, first-time diamond buyers may want to seek out gemologists for assistance.

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Learn the fundamentals of diamonds — from optical and physical properties to grading and ethical and legal issues. Learn to distinguish natural diamonds from synthetics and diamond simulants. Keep the kit, which includes one natural, jewelry-grade diamond. If you successfully pass the quiz and gem identification test, then you will be an IGS-certified Diamond Specialist.
marquis cut diamond ring
Silver engagement ring, palladium ring with a marquise-cut diamond and cluster of smaller "Dot Style" diamonds to the side. Photo by Lorraine Hughes, Licensed under CC By 2.0. You can also get something similar at

Gemologists grade diamonds into dozens of categories. Although this helps professionals in the gem trade, the average consumer can find this information confusing. Young couples looking for their first diamond engagement set may find this especially troublesome. While they want to gather enough information to make an intelligent decision, all that data may overwhelm them.

To help consumers make informed decisions, I've divided this article into two parts. First, I'll explain diamond rating and what qualities have the greatest impact on diamond grades. Once you understand that, I'll then have recommendations to help you choose a diamond.

1.51-ct princess-cut diamond engagement ring
A 1.51-ct princess-cut diamond (G/VS) and 2 0.30-ct trillions (G/VS) in a platinum setting. Photo by ilovebutter. Licensed under CC By 2.0. You can also get something similar at

Diamond Rating: What Qualities Matter Most?

You've probably heard of the 4 Cs of diamond grading. I've ranked them below, in terms of importance when you choose a diamond.

  • Cut: The most important property.
  • Color: The second most important property.
  • Clarity: The third most important property, provided the grade is "SI" (Slightly Included) or better.
  • Carat: The least important property.

Now, we'll go through the evaluation of each property.

The Cut

One of the hardest properties to judge, the cut is nevertheless the most important. Choosing a well-cut diamond requires paying careful attention to a few key characteristics.


Brilliance refers to the total amount of light returned by the gem. A cut gem's pavilion facets should act as mirrors, reflecting the light entering the stone back to the observer. Diamonds have a high refractive index, which contributes to their great brilliance. However, the cut plays a critical role.

The angles at which a diamond is cut affects greatly how efficiently it reflects light. This then impacts how well the diamond sparkles, its scintillation. The ideal angle for diamond pavilion facets is 41°. This is usually quite convenient, due to the octahedral shape of a standard diamond crystal. Unfortunately, diamond cutters can't cut all mined diamonds to excellent proportions. Often, they must compromise between maximum brilliance and maximum yield (weight/carat retention). If cutters remove too much material from the diamond rough, they may cut away their profit. Thus, for economic reasons, many diamonds receive a cut with less than ideal proportions.

A little "cheating," finessing facets so they meet where they should, has no great impact on the stone. However, if a cutter varies a little further from the ideal cut angles, the diamond's brilliance begins to suffer. If the cutter varies too far, you'll get a diamond that just doesn't compare to others in brilliance or fire.

The Make of a Diamond

The Gemological Institute of America (GIA) and other gem labs assign cut grades to round brilliant-cut diamonds. Despite this, no grades on any report can effectively evaluate the cut.

Most jewelers, familiar with correct proportions, can judge the cut from the shape of a diamond. In the jewelry trade, they call it the "make" of a diamond. A "heavy make" means the stone was cut to retain weight at the expense of a good cut. This was most likely done to maximize the price. A "spready make" means the stone appears, "face-up," to weigh more than it actually does. Such a make can sometimes impact the cut as well.

For most of us, simply comparing gems side by side serves as the best test. If you have two diamonds of the same grade, but one is significantly brighter, the cut makes the difference.

Non-Round Diamond Cuts

marquis cut - diamond ratingPlease understand, the information above assumes we're talking about round diamonds. Cutters can cut all their major facets at the same angle due to the round's symmetrical proportions. For other shapes, this doesn't hold true.

For example, many people prefer a marquise shape. This is fine, but don't expect a marquise, or any other shape, to have the brilliance of a round. A marquise requires cutting a number of facets just to accommodate the shape. Cutters cut these facets at angles that vary (slightly to greatly) from those that give the greatest brilliance. This is a simple fact: the more facets cut at the ideal angle, the greater the gem's brilliance.

Single Cuts

When looking for diamonds, you may come across the terms Single cut," "Old Mine cut", or "Old European cut." These gem cuts have only eight facets running from the girdle down and eight up to the table facet. That makes a total of 17 facets. In contrast, a standard round brilliant cut has 57 facets. In contemporary jewelry, you'll usually see single cuts used on small accent stones. Occasionally, you'll find an older diamond of decent size with this cut. Obviously, these gems won''t have the brilliance of a full-cut diamond. Therefore, they typically aren''t worth as much.

single cut and round brilliant cut
Compare the number of facets of a single cut to a round brilliant.

Diamond Shape

Another factor to consider when you choose a diamond is its shape. An ideal-cut diamond should be symmetrical, not lopsided. (This point may seem obvious. Sometimes, however, it helps to point it out). A poorly proportioned gem can be "camouflaged" in its setting. Thus, you might not notice it until after you've bought it. Always examine a diamond loose, not in the setting.

Girdle Proportions and Brittleness

diamonds do breakIn terms of resisting scratches, diamonds are famously the hardest material in the world. However, in terms of resisting blows, they're brittle. (Read our article on gemstone hardness and wearability for more information). This property affects the cut evaluation, particularly when examining the girdle.

A gem's girdle is its widest part when viewed from the top but its thinnest when viewed from the side. If a gem cutter cuts the diamond's girdle too thin, it can present a weak area, just asking for trouble. Some girdles get cut to a knife-edge level of thinness. Definitely avoid this, because it can cause the gem to fracture or break.

To get an idea of what constitutes normal proportions for a diamond girdle, look at the previous illustrations for a single-cut and round brilliant diamond.

Diamond Color

The closer the stone's color is to colorless, the greater a white diamond's value.

GIA Diamond Color Grades

Before the introduction of the GIA color grading system for white diamonds in the 1950s, various systems used the letters A, B, or C. To avoid confusion, the GIA system begins with the letter D and goes to Z.

D, E, and F: The highest grades for white diamonds. These colors are described as colorless."

Color D diamond - diamond rating
Round diamond, 0.19-ct, color D, clarity SI2, very good cut. Photo © James Allen. Used with permission.

G, H, I, and J: These colors are described as "near colorless" or "white."

color G diamond
Round diamond, 0.20-ct, color G, clarity VVSI1, excellent cut. Photo © James Allen. Used with permission.

K to Z: These colors are described as tinted, usually yellow or yellowish brown. Diamonds with colors K, L, or M are often said to "set white." That means, if jewelers set them in yellow gold, the stones will appear white, since they have only slight tints. However, white gold or platinum settings would set off the tinted colors.

K color
Round diamond, 0.30-ct, color K, clarity SI2, excellent cut. Photo © James Allen. Used with permission.

The further the grade goes down the alphabet, the stronger the tinting and lower the diamond's value. However, once you get to the extreme end and the color becomes richer, you have a fancy colored diamond. No longer considered "off colored" white diamonds, their value starts going up again. Fancy colored diamonds have their own GIA grading system.

yellow diamond ring
Fancy yellow diamond ring, 0.85-ct fancy yellow diamond, brilliant-cut champagne diamonds, and 750-yellow gold. Photo by Mauro Cateb. Licensed under CC By 2.0. You can also get something similar at

Color Grading and Color Ranges

Gemologists color grade a diamond by placing it next to a set of previously graded gems. Then, they compare its color to the graded gems to find the closest match.

Although this low-tech approach is accurate, it's also expensive and time consuming. With smaller gems, gemologists sometimes compromise by grading batches within a range rather than assigning specific grades. Thus, you may find diamonds under a carat graded as GH, IJ, etc.

Grading small diamonds in this manner provides meaningful information and saves you quite a bit of money. It costs over $100 to accurately grade a diamond. For a large diamond, where subtle differences in grades can make a significant difference in price, this is worthwhile. For the majority of stones, however, this isn't cost effective.


The size and number of inclusions inside a diamond determine its clarity. An inclusion can be another mineral, a fracture, or occasionally a void. Simply put, anything that will interfere with the free passage of light in a gem qualifies as an inclusion.

Diamond Inclusions

Clarity Grading

Just as with color, there are many possible clarity grades. Gemologists judge a diamond's clarity based on what they can see at 10X magnification, under ideal conditions.

The highest grade a diamond can get is Flawless. That means you can see no inclusions at 10X magnification. However, this doesn't mean you can't find inclusions with higher magnification. Also, don't assume it's the only grade with no inclusions visible to the naked eye.

Diamond Clarity Grades

In addition to F for Flawless and IF for Internally Flawless, clarity grades use the letters V, S and I. They stand for Very, Small, and Inclusion. Numbers 1 to 3 further designate levels within grade.

After F and IF, clarity grades proceed as follows:

  • VVSI1 (Very, Very Small Inclusions One)
  • VVSI2 (Very, Very Small Inclusions Two)
  • VSI1 (Very Small Inclusions One)
  • VSI2 (Very Small Inclusions Two)
  • SI1 (Small Inclusions One)
  • SI2 (Small Inclusions Two)

All these clarity grades indicate diamonds with no "inclusions visible to the naked eye. These are all "eye clean."

Further down the grading scale, you'll find I1 and I2 diamonds. These have eye visible inclusions. However, they're still considered gem grade.

I1 Grade Diamonds - diamond rating
I1 Grade Diamonds

When is a Diamond Not a Gem?

You may encounter diamonds with clarity grades P1 or P2. (P stands for piqué, French for "pricked" or "marked"). They're not usually considered gem grade, because so little light will pass through them. However, since they still have the magic name "diamond," they do show up on the market regularly.

Before you choose a diamond, beware of ads promoting a "1-carat diamond ring for a very low price." Not all diamonds are considered gems. After all, "gem" is a subjective term for a mineral desired for adornment due to its beauty and durability. (It's so subjective, even some popular gems fall short of that definition). Nevertheless, the vast majority of mined diamonds are usually considered industrial grade. In fact, they're used as abrasives. Many industrial grade diamonds, P1 and P2, find their way into jewelry simply due to diamond's advertising appeal.

Editor's Note: In the World Jewellery Confederation's (CIBJO) diamond clarity grading system, as well as others, the P1 and P2 grades are equivalent or overlap with the GIA's I1 and I2.


By far, carat or weight is the easiest quality factor to understand when you're shopping for diamonds. Simply put, small diamonds are more common than large ones, but larger stones are more desired. Therefore, smaller diamonds cost less per carat than large ones. This explains why you'll encounter jumps in price per carat as diamond sizes increase.

If you look at a diamond broker's price list, you'd see these price increases with size. For example, for a given grade of diamond, say you find a price of $900 per carat for a ½-carat size. A diamond of the same grade at ¾-carat might cost $1,100 per carat. At a full carat, a diamond in the same grade could cost $4,000 per carat!

half-carat stone
A half-carat diamond held by beeswax. Photo by Mauro Cateb. Licensed under CC By-SA 3.0.

Diamond Rating and Choosing Your Stone

For those readers ready to choose a diamond, I recommend stones in the white range of color (G, H, I, or J) with clarity grades SI1 or SI2. Visually, such stones will be bright and lively. They'll make wonderful diamond jewelry.

Can You Get Better Value with a Lower Diamond Rating?

Occasionally, looking for a lower clarity graded diamond may be worthwhile. This depends on the individual diamond and setting. Sometimes, the "eye-visible" inclusion that made it rank lower may have an insignificant effect on its appearance. Overall, the stone may still look delightful.

However, going down further in quality in other areas is rarely worthwhile. Many jewelers make their living by supposedly underselling the competition. In fact, they just sell lower grade gems. Without making a comparison with better quality diamonds, customers often convince themselves they've found a great deal. When they make the comparison later, they find disappointment.

Does Size Matter?

Selecting a diamond is a personal thing. Of course, not everyone will have the same opinions. However, consider this observation carefully before you choose a diamond. In my experience, most folks will enjoy a smaller, higher quality dazzler more than a larger but mediocre gem.

gold wedding ring
Gold wedding ring with small diamond. Photo by Donald Trung Quoc Don (徵國單). Licensed under CC By-SA 4.0. You can also get something similar at

Some Diamond Rating Differences Matter More to Appraisers (and Sellers)

People who take their diamonds seriously look at a lot of them. They get a kick out of finding those rare stones, those nearly colorless or nearly clean under magnification. These rarer gems do command higher prices. Still, that doesn't mean they're prettier or that the ordinary consumer will enjoy them more.

Let's say you set two well-cut diamonds side by side. One has grades of D and VVSI1.

Color D, clarity VVSI1
Round diamond, 0.71-ct, color D, clarity VVSI1, excellent cut ($3,030). Photo © James Allen. Used with permission.

The other, G and SI1.

Color G, clarity SI1
Round diamond, 0.71-ct, color G, clarity SI1, excellent cut ($2,180). Photo © James Allen. Used with permission.

With the naked eye, you wouldn't see much difference, if any. However, you'll likely have a strong emotional reaction when you compare their prices.

The point is simple. These are the rarest quality diamonds. Only a sophisticated diamond appraiser who inspects them carefully under magnification will appreciate their differences.

Differences Between an SI1 and an SI2 Diamond

Occasionally, you'll notice a difference between a diamond clarity graded SI1 and SI2. (SI1 means the diamond has small inclusions, somewhat easy to find. SI2 means the diamond has small inclusions, easy to find).

When I look at a diamond graded SI1, I usually see something like the illustration below, left. Those one or two tiny dark spots have no effect on brilliance. On the other hand, diamonds graded SI2 may show three different conditions. They may contain larger inclusions. Sometimes, as shown in the illustration below, right, they have inclusions placed near the center (and thus more visible) or simply have many more of them.

SI1 and SI2 clarity comparison

Some diamonds graded SI2 will have no significant difference in brilliance when compared to a SI1 stone. However, in extreme cases where the stone has many inclusions, they may take up 5% or more of the visible area. Although none of these inclusions are large enough to see without magnification, they'll still reduce brilliance by 5% or more.

Diamond Rating: Conclusion

To summarize, when you choose a diamond, go with quality rather than size. Also, keep in mind that the differences between the rarest quality diamonds may not be worth the price (or even perceptible to anyone but an expert).

If you're buying online, be sure to use retailers like James Allen and Blue Nile, where you can see your diamond in 360° before you buy.

And, if you haven't settled on diamonds yet, remember, colored gemstones can make excellent alternatives.

vintage engagement ring
Vintage diamond and sapphire engagement ring. Photo and ring by Petragems. Licensed under CC By-SA 4.0. You can also get something similar at

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

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