How Are Diamonds Graded and How to Check Diamond Quality Yourself?

Which Diamond Grading Factors Are the Most Important?

The most important quality characteristics that are graded in a diamond are its color, clarity, and cut.

Diamond carat weight is measured in carats and, like the above three attributes, is also an important determinant of diamond price.

There are different grading scales for diamond color, carat, and cut, but for this article, we will use one of the most popular standards – the system introduced by the GIA (Gemological Institute of America).

On the GIA scale, diamond color is graded by using letters – from D (the highest grade) to Z (the lowest grade).

The GIA clarity scale consists of the following grades (from highest to lowest): IF, FL, VVS1, VVS2, VS1, VS2, SI1, SI2, I1, I2, and I3.

The cut grading scale used by the GIA includes the following grades: Excellent, Very Good, Good, Fair, and Poor.

Let’s look at each of these characteristics in more detail.

Diamond Color Grading Scale

Different diamond grading labs use different color grading scales, and one of the most popular is that of the GIA (Gemological Institute of America).

In this system, color grades are denoted by letters, starting from D (the highest color grade), with the lowest grade being Z.

On the GIA scale, stones graded with letters from D to F are referred to as Colorless.

The next group contains the Near Colorless diamonds, with grades from G to J.

Stones graded in the K-L-M range are called Faint Yellow.

Grades from N to R are assigned to the Very Light Yellow stones, and diamonds graded from S to Z are referred to as Light Yellow.

The color of a diamond is usually graded by comparing it with a set of benchmark stones (master stones), each of which corresponds to a certain color grade.

For example, if a stone has more color than the master diamond graded F but less color than the one graded H, then the diamond will most likely be classified as color grade G.

Diamond Clarity Grading

Diamond clarity is graded by inspecting the stone under magnification as well as with the naked eye and determining how clear the diamond is of visible flaws, or inclusions.

Inclusions can be internal or surface flaws, and their visibility depends on several factors: number, size, position, and color.

So, a stone can have many inclusions, but if they are visible only with a microscope, the diamond’s clarity grade will be relatively high.

On the other hand, a diamond can have few inclusions, but if they are big and visible with the naked eye, the stone’s clarity will be relatively low.

Also, centrally located inclusions impact clarity more, as they are more visible than flaws towards the sides of the stone.

The GIA grades diamond clarity with the following grades: IF (Internally Flawless), FL (Flawless), VVS (Very Very Slightly Included), VS (Very Slightly Included), SI (Slightly Included), and I (Included).

All these grades, except IF and FL, are further divided into finer grades by adding numbers to them, so in practice the scale becomes: IF, FL, VVS1, VVS2, VS1, VS2, SI1, SI2, I1, I2, and I3.

Diamond Cut Grading Standards

Cut is graded by comparing a diamond’s proportions with a set of benchmarks.

These benchmarks are optimal diamond proportions that have been found to enhance some of the stone’s important characteristics such as brilliance and sparkle.

For example, some of the key ratios used to grade the cut of round diamonds are the depth of the stone and the diameter of its table expressed as percentages of the diameter of the girdle (the diamond’s widest edge).

The GIA evaluates round cuts on a scale that includes the following grades: Excellent, Very Good, Good, Fair, and Poor.

Other grading labs have scales with slightly different names, but the logic behind these systems is the same.

In general, you can expect brilliant cuts (such as the round cut) that are more highly graded to have greater brilliance, whereas lower-graded cuts will look less bright.

How Is Diamond Carat Calculated?

Carat is a unit of weight; one carat is equal to 0.2 grams, or 200 milligrams.

To find out the carat weight of a diamond, you can weigh it in grams and then convert the value to carats by dividing by 0.2. For example, a 0.5-gram diamond has a carat weight of 2.5 carats.

To convert the carat weight of a diamond to grams, simply multiply by 0.2. Thus, a 1.5-carat diamond weighs 0.3 grams, or 300 milligrams.

Diamond carat weight is an important price factor – since bigger diamonds are rarer, they are exponentially more expensive per carat than smaller ones.

Additional Diamond Grading Characteristics

Apart from the 4 Cs, there are other characteristics that can be included in a diamond certificate, such as symmetry, polish, light return, fluorescence, etc. Let’s take a look at some of the more important ones:

Polish

Although polish is not part of the 4 Cs and can be considered a secondary quality characteristic, it still contributes to the appearance of a diamond.

Stones with top-graded polish have a smooth surface that lets light enter the diamond easily and produce brilliance.

Diamonds with poor polish have a rough surface that obstructs light entry and thus makes the stone look less brilliant.

Symmetry

This quality characteristic refers to how symmetrical a diamond’s facets are as well as how well aligned the different parts of the stone are with each other.

No diamond is cut perfectly symmetrical, but asymmetry becomes a problem when there is obvious misalignment of facets or irregularity of their shape.

Fluorescence

Fluorescence is the ability of the diamond to emit a glow (usually blue) when exposed to UV light.

Diamond reports usually include information on whether a stone exhibits fluorescence as well as how strong it is.

Girdle

The girdle of a diamond is actually its edge, or the widest part of the stone (in round cuts). The way the girdle is usually graded is by simply stating how thick or thin it is.

In general, girdles that are too thin are undesirable as they can get chipped easily. Extremely thick girdles are also problematic because they can make the stone look disproportional.

Culet

The culet is the facet at the bottom of a diamond and is especially common in round cuts. A diamond can have a culet, or it can have none, in which case the stone’s bottom will simply be pointed.

Diamonds with no culets can be at an increased risk of chipping at the bottom, unless it is well protected by the setting.

While some jewelers argue that having a culet is more desirable, if the culet is relatively big, it could be too visible through the stone’s top.

How to Grade a Diamond Yourself

Sometimes you need to determine the quality of a diamond, but you don’t have a grading certificate for it. In such a case, you can still grade the stone, but the grades you will come up with will be approximate and are best expressed in terms of grading ranges instead of exact values. Let’s see how you can do that.

(Note: This is applicable to round cut diamonds mostly and is less relevant to other diamond cuts.)

How to Grade Diamond Color

Diamond color is graded by comparing the evaluated stone with a set of graded master stones and determining which one of them is closest to the assessed diamond in terms of color.

If you don’t have access to diamonds that you can use as benchmarks to compare your stone against, you can try to determine its approximate color grade by looking at how colorless it looks.

If your diamond doesn’t appear to have a visible yellow tint, then the stone color grade will most likely fall in the D-E-F-G-H range. It’s hard to pinpoint the exact grade further without using graded stones for comparison.

Diamonds that have a barely visible yellowish hue most likely belong to the I-J-K-L-M grading range.

Stones that have some yellow color that is very light but nonetheless visible would likely be graded in the N-O-P-Q-R range.

Diamonds with a clearly visible yellow tint are usually graded with one of the color grades in the S-Z range.

How to Grade Diamond Clarity

Diamond clarity is graded depending on how many natural flaws can be seen within a diamond or on its surface by looking at the stone under magnification or with the naked eye.

If you look at a diamond using a 10x loupe and cannot readily see any visible flaws inside, then the stone’s clarity would most likely be graded in the FL/IF/VVS1/VVS2 range.

The difference between the FL/IF and VVS1/VVS2 grades is that the latter contain some tiny inclusions, but even trained graders cannot spot them easily.

If the diamond you are looking at has some inclusions that are visible under magnification but not to the naked eye, then the stone’s clarity is most likely in the VS1/VS2/SI1 grading range.

Flaws in SI2-clarity diamonds may or may not be visible to the unaided eye, but if they are, they will usually be located towards the sides of the stone.

If the diamond has inclusions that are easily noticeable with the naked eye, then the stone is most likely an I1/I2/I3-clarity diamond.

These stones are more likely to have visible inclusions that are centrally located – i.e., towards the center of the table when the stone is looked at from the top.

As you can see, it is hard to pinpoint the exact clarity grade of a diamond unless you are experienced in gemstone grading.

How to Grade Diamond Cut

To grade the cut of a diamond, you need to evaluate some of its measurements and proportions that are considered important for how the stone reflects light.

Grading cut is a complex process, but you can determine approximate cut quality by looking at some key ratios and characteristics.

Table Percentage

One of these important ratios is the diamond’s table percentage, which is calculated by dividing the diameter (for round cuts) or width (for rectangular cuts) of the stone’s table (the topmost facet) by the diameter or width of the girdle (the widest part of the stone).

For round cuts, a good table percentage is in the 52%-60% range. Values between 53% and 58% are considered ideal. Values below 51% or above 64% are seen in cuts graded Fair or Poor.

For rectangular cuts, the table percentage value should ideally be in the 60%-70% range. Proportions that deviate significantly from these values are not desirable.

Depth Percentage

Another important ratio in cut grading is depth percentage, which is calculated by dividing the stone’s depth (distance from the top to the bottom) by the diameter or width of the girdle.

A good depth percentage for a round cut is in the 57%-64% range, ideally between 58% and 60%.

For most rectangular cuts, an acceptable depth percentage is in the 60%-70% range, but exact desirable ranges vary depending on the particular cut examined.

Girdle Thickness

An important cut characteristic is girdle thickness.

Regardless of cut, the ideal girdle is thin to thick, with extremely thin or extremely thick girdles being undesirable.

Culet Size

Another feature you should pay attention to is the culet – the facet on the very bottom of the diamond.

Ideally, the culet should be small or there should be none. Big culets are not recommended, as they are too visible through the stone’s top.

Estimating Diamond Cut Quality

There are more details that are considered in cut grading, but you can use the above characteristics to make a quick estimate of cut quality.

The rule of thumb is that if all of these characteristics are in their recommended ranges for a particular stone, then its cut can be graded anywhere from Good to Excellent (these grades are most applicable to round diamonds).

The more of the above characteristics fall outside of their recommended ranges for a given cut, the more likely it is to be graded Fair or Poor.

How to Determine Diamond Carat Weight

Carat weight is easy to calculate for a given diamond. Since 1 carat is equal to 0.2 grams, all you have to do is weigh the stone and divide its weight in grams by 0.2.

For example, a diamond that weighs 0.35 g has a carat weight of 1.75 ct (0.35/0.2).