To capture a diamond’s true beauty, the most important C is the diamond’s cut. Unfortunately, it is also the most misunderstood of the Four C’s. There are a lot of reasons for this, first, the other C’s are more obvious. Choosing Color means choosing a stone somewhere on the spectrum between a totally colorless stone (D color) and a pale-yellow diamond (Z color); choosing Clarity is about choosing a stone base on the number and size of the imperfections inside, and choosing Carat Weight is the easiest of all, it’s simple math,  the weight of the diamond under consideration.

Understanding the diamond Cut grade requires some work. Not to worry, if explained properly, understanding Cut is not difficult. Let us break it down for you.


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An excellent cut stone displays incredible brightness, fire and sparkle. To add to the confusion brightness is often called brilliance and sparkle is often called scintillation. They mean exactly the same thing. From here on out we’ll (bracket) these terms with the other regularly so you don’t get confused. An excellent cut stone also has very specific proportions that maximize these light performance descriptors, and an excellent cut stone adds nicely to the diamond’s overall physical appearance. Each is discussed below.

Almost all diamond professionals agree that Cut is the most important C for the overall beauty of your diamond. If you’re going to balance cost versus benefit, this is not the area to save money.

Cut grade is also the most affordable at the highest end of the grading scale because cut is the only one of the 4 C’s that is “manmade” – meaning that a skilled cutter can deliver an “Excellent” cut (the very top of the GIA grading scale) stone much of the time; however, Mother Nature determines Color, Clarity and Carat size and she is very stingy at the top end for these other 3 C’s. For example, the best diamond color – D (totally colorless) exists in less than one out of 5000 diamonds; however, the best diamond cut – Excellent (in GIA terminology) exists in about 1 out of 25. This means, all else being equal, a D colored stone is 200 times rarer than an excellent cut stone.

The same can be said for a “Flawless” clarity rating (ironically, still about 1 in 5000) and finally, the same can be said of a very large carat weight stone (i.e. nature does not usually make them that big, meaning one five carat diamond (in the rough) is incredibly difficult to find, but five one carat diamonds are much easier. You get the point – rarity = $$$$$.

What is an Excellent Cut Diamond?

First we’re going to talk about what an “Excellent Cut” round brilliant diamond looks like below, then we’ll talk about Cut’s little brother and sister: Symmetry and Polish.

Cut falls under the category of a diamonds “proportions” and Polish and Symmetry fall under of the category of a diamond’s “Finish.” So, when a jeweler says the diamond “has great proportions and finish,” this is what he is talking about. Cut, Symmetry and Polish each has its own separate rating on a GIA diamond certificate and once you know how simple Cut, Symmetry and Polish are to understand, you will know what a beautiful diamond looks like on a diamond certificate, and more importantly you’ll be able to find the diamond of your dreams quite easily, online or on foot.


Let’s Start Now with Cut

A diamond’s Cut grade comes in: excellent, very good, good, fair and poor. Cut is a measure of the diamond proportions. An “Excellent Cut” grade is to diamonds what the “Ferrari 458 Italia” is to sports cars. You may know nothing about sports cars, but when you see that amazing machine on the street your jaw just drops to the floor – you know it’s hot. The same is true with a diamond that has an Excellent Cut.  What are the “Ferrari 458” (Excellent Cut) proportions of a round brilliant diamond? They are*:

  1. Table size: 52-62%
  2. Crown angle: 31.5-36.5 degrees
  3. Pavilion angle: 40.6-41.8 degrees
  4. Girdle thickness: thin to slightly thick and consistent all the way around
  5. Total depth percentage: 57.5-63%
*Source: The Gemological Institute of America, Cut Grade Reference Chart.

Each is described in detail below…

First things first, understand that getting #1, #2, and #3 above right are very important for the most appealing proportions and optimal light performance. However, an excellent cut diamond will also deliver #4 and #5.

Table Size (very important) – The table is the flat part on the top of a round-brilliant diamond. It is measured as a percentage of the overall maximum width of the diamond (called the average girdle diameter). Table sizes below 50% or above 66% of the average girdle diameter are suboptimal for light performance. Too small and there is not enough flash, too big and there is too much flash (overpowering other light features such as fire and scintillation).

Crown Angle (very important) – This is the angle of the crown as it relates to the girdle. If the girdle plane is zero degrees. The Crown Angle should be between 25 and 35 degrees for optimal light performance.

Pavilion Angle (very important) – This is the angle of the pavilion as it relates to the girdle. If the girdle plane is zero degrees. The Pavilion Angle should be between 40.6 and 41.8 degrees for optimal light performance.diameter.

Girdle Thickness (important) – On a round-brilliant diamond, the girdle should be thin to slightly thick, and bow inward (i.e. get slightly thinner) between where the facets’ straight edges (called the points) meet the girdle. Thickness is measured as a percentage of the average girdle diameter, but is also easy for a grader to eyeball.

Total Depth Percentage (somewhat important) – This is the distance from the table to the cutlet, expressed as a percentage of the overall girdle diameter. A total depth percentage of 57.5 to 63% is optimal.

Why is there so much variance in the numbers above?

The funny thing about diamonds is that while each individual measure has as much as a 10% swing in the “Excellent” range above, any one outcome at the extremes might make it impossible for another to fall in its own “acceptable” range and maintain an Excellent, or even Very Good cut grade. For example, if a total depth percentage is 63%, getting a crown angle of 31.5-36.5 degrees, while simultaneously maintaining a pavilion angle of 40.6-41.8 degrees is difficult if the girdle is even a little too thick, (and it might even require a very thin girdle to accommodate). This means that most measures are in the middle of the highlighted ranges above on an Excellent or Very Good cut diamond.

Fortunately, there is no need to assess the interplay between these yourself (unless you like that kind of thing), look for an Excellent Cut rating on the GIA certificate (available with one click at James Allen and Blue Nile) if you want a diamond that is approximately right in these areas and you will know that your diamond is the “one in 25.” Further, if you are willing to move down to Very Good, the diamond will be almost as brilliant, scintillating and fiery, and you can save some money.

What does Ideal Cut, Signature Ideal, True Hearts, etc. Mean Then?

Jewelers sometimes give their own cut grades to diamonds that they feel are super special. These labels indicate that they have put their own stamp of approval on the diamond in addition to what GIA has indicated. They sometimes given them then own non-GIA terms such as: Signature Ideal Cut, True Hearts, etc. Most of them have one thing in common. If you go to their certificate, if it is a GIA graded diamond, they almost always have an Excellent Cut rating.

There is no evidence that these alternative labeled cut grade diamonds are better than GIA’s ratings. We feel that these cut terms are more useful when trying to compare diamonds across different diamond grading labs at stores that sell more than just GIA graded stones (most do). For example, AGS’s top end cut is called is Ideal. EGL’s is Ideal Plus and GIA’s is Excellent. There is also no evidence that an EGL Ideal Plus grading is better that a GIA Excellent grade. A lot of it is branding. In the end if you are looking at a diamond, we recommend that you stick with GIA graded diamonds, then read their GIA Diamond Grading Report to determine Cut.

Two Very Important “Non-C’s”

The GIA Cut Grade will not receive an Excellent if either the Symmetry or Polish Grade are more than one grade below (below Very Good).  Cut encompasses the overall diamond while Symmetry and Finish capture specific aspects of at the diamonds. Both are discussed below.

Symmetry – Finish measure #1

Imagine driving down the road, seeing the Ferrari 458 Italia discussed above sitting at a stoplight coming the other way. While the beautiful machine is stopped, waiting for the light to turn green you notice the car’s tiny spare tire mounted on the front right wheel. The right side would dip a little more than the left and the thinness of the right front tire would not match the flat, sexy, 11.6” wide X 28.1” tall racing tires mounted on the other three wheels. Even if the center wheel color and design was exactly the same as the other three, the sports car proportions would just look off. In diamond-speak the Ferrari would receive a “Poor” Symmetry rating because everything was not absolutely even and uniform.

Symmetry Grade. Like the cut grade, a diamond’s symmetry grade comes in: excellent, very good, good, fair and poor.

A perfectly symmetrical diamond has all of its parts in perfect proportion. The table is perfectly centered, the girdle is perfectly round, the facets all point properly, the crown and pavilion facets are aligned, the table is a perfect octagon shape, there are no extra facets and the outline is completely even.

The reality is that no diamond is perfectly symmetrical, not even diamonds that carry and Excellent Symmetry grade; however, the variances are nearly impossible to discern at the high end of the grading scale for all but a skilled diamond grader in a lab environment.

Polish – Finish measure #2

Back to the Ferrari. As the light turns green and the vehicle approaches, you gawk at it (hey you’re human – who wouldn’t!). You are expecting a smooth, sexy, flowing, shiny, triple waxed Alpine White paint job, but instead you notice that the finish looks like someone took a Brillo pad to it. It has abrasions, nicks, scratches and pits. While the outline of the car looks great from afar (except for that weird spare tire!), when you stare at the sexy machine slowly approaching, your inner fantasy is interrupted by marks of all kinds on the outside. In diamond-speak the Ferrari would receive a “Poor” Polish rating.

Like both the Cut and Symmetry grades, the Polish grade comes in: excellent, very good, good, fair and poor.

The Diamond’s Light Dance (Brightness, Fire and Sparkle)

Excellent cut stone displays maximum brilliance (brightness), fire and scintillation (sparkle).

Very Good cut stones are one step lower, but the brilliance (brightness), fire and scintillation (sparkle) are also quite impressive.

We don’t recommend going below the Very Good cut grade. The savings is not worth the decrease in light performance.

How does light performance work?

When light hits a diamond it either reflects off the surface or enters the stone. When it reflects off the surface, the light that you see is white light. In diamond parlance, it is called brightness (brilliance).

When light enters the stone, the diamond acts as a prism and breaks the light down into the colors of the rainbow before sending it back up through the table to the eye or out the sides or bottom. This type of light is called fire because of its multiple colors. The better the cut grade the more that is reflected up through the table, instead of out of the sides or bottom.

When the diamond, the light source, or the observer moves the result is a diamond that sparkles (scintillates) with white and rainbow light.

A well-proportioned stone displays great brightness, fire and sparkle. While one that is too shallow or too deep has light leakage. In the extreme, a round diamond whose pavilion is too shallow has something called the fisheye effect – an unattractive gray ring under its table. When the pavilion is too deep, light leaks out of the bottom and the center is dark. This is called a nail head diamond. Neither of these issues exist on an Excellent or Very Good cut diamond.


Diamond Light Show

Polish and Symmetry add to the light show, but also give the stone beauty in its own right.

Given the cost of Cut, Polish and Symmetry compared to the other 3C’s we recommend acquiring a diamond with an Excellent Cut rating, an Excellent Polish rating and an Excellent symmetry rating. Blue Nile and James Allen are our top two online diamond store winners for 2017 and they both offer a great selection of Excellent/Excellent/Excellent graded diamonds for Cut/Polish/Symmetry at 20 to 40% off retail prices. To see their 1 and 2 Carat selection, click WhiteFlash or James Allen.