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Calculating Diamond Proportions (Geeking Out Like a Pro)

Unfortunately, some diamond dealers like to describe their stones as having:

  • "Really nice make."
  •  "Fine make."
  • "Excellent/Ideal cut proportions."

While the cut quality of their rocks are nowhere close to being ideal by our standards.


 Hope Diamond, courtesy Smithsonian.

Although this may be true, there is no reason for despair. After all, Pandora slammed the box closed on the hands of those charlatans before hope escaped.

No, not the Hope Diamond, (<----- look to the left) just plain old Hope!

Inasmuch as buying a diamond can seem like the blind leading the blind, it's easy to feel lost.  I'm going to teach you how to calculate diamond proportions for yourself, so you don't get taken advantage of.

At the same time, I'll show you how to estimate light return and the balance of brilliance and dispersion.


At some point you might realize that AGS Ideal and GIA Excellent cut is falling short of your expectations. Because the spectrum of proportions allocated for those two cut grades is significantly broad. That's okay, because I'm going to show you how to find the sweet spot that produces the highest volume of light return.


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How to Calculate Diamond Proportions:

Be the first kid on your block to do it!

This article contains the “Secret Sauce” of the McDiamond Industry!

Nerd emoji with tongue.

It's the recipe for diamond buying success!  Not to mention everything you need to feel confident buying an engagement ring. Don't worry, you're going to dazzle her.

You might even be able to hold your own in a room full of jewelers! That is to say, you'll be able to walk the talk and walk the walk like a pro.

The first thing to remember is that the only way to accurately determine the overall proportions rating of a diamond is to know all the contributing factors.

At the same time, you'll learn how each section of a diamond contributes to light performance. Plus, you'll discover how to maximize light return and minimize leakage. Let the games begin!

 Pro Tip:

Diamond proportions dictate the volume of light return, as well as the balance and intensity of the sparkle.  White sparkle is commonly referred to as brilliance, while dispersion refers to the colored sparkle or fire.

You're going to rock the ring once you know the right combination of angles and degrees to search for. After all, if you stop to think about it, a diamond is a three dimensional model. Which means that we can use mathematical ray tracing to estimate the volume of light return and sparkle factor.

What You'll Need To Calculate Diamond Proportions:

  • Minimum/maximum diameter.
  • Overall depth measurement.
  • Table diameter.
  • Total depth.
  • Crown angle.
  • Pavilion angle (the most critical factor).
  • Girdle thickness.
  • Culet size.

Don't worry if you don't have these measurements or even know what they represent. By the time you finish reading this article you'll be a diamond buying pro! Not only will you understand what the measurements represent, but you'll also be able to calculate them for yourself.

With that in mind, whip out that slide rule because we've got work to do!

Lab Graded Diamond Proportions:

Act One – Scene One:

Character: An analytical type who looks a lot like you.
(i.e. Mathematician, Engineer, Computer Programmer, Geek, etc.)
To be read: slowly and without feeling.

Calculating Proportions of "Certified Diamonds":

All right, first things first.

It's a common misconception that buying a “certified diamond” means that you're receiving something of high quality.

After all, the diamond (regardless of quality) is accompanied by an official looking document. Which states that the overall cut grade is excellent, so it must be good, right?

Don't worry, we'll get to that in a moment. But for now you should know that many jewelry salespeople refer to the certificate in hopes that you'll assume that the diamond is high quality.

This is known as transference of authority in psychological circles and might look something like this:

"This diamond is certified by the Gemological Institute of America, so that you know the quality is Excellent."

In light of this practice, let's begin with the understanding that there's no such thing as a certified diamond.

 The Hoax, the Lie, the Myth, the Truth About Certified Diamonds:

As has been noted, certified diamonds do not exist.

Quite simply, there is no such thing as a diamond grading certificate.

Although this may be true, millions of people buy into the idea that gemological labs certify diamonds. In the same way that people claim to have seen Big Foot, or having been abducted by aliens.

Mobile Monkey Chatbot.

At the same time, you've got people like Larry Kim, who sold his company Wordstream for $150 million. The guy loves unicorns!

How do I know this? Just look at the banner used to promote his Facebook messaging service Mobile Monkey. Which is totally awesome by the way! The chatbot revolution is upon us!

Well, at least that's real. Whereas that pesky Mexican Chupacabra that my girlfriend's mother is always obsessing about is not. Regardless of the overwhelming testimony given by Mulder and Scully to its existence.

P S. My girlfriend would like you to know "My mother doesn't believe in that!" 

Perhaps something got lost in translation? "Mi español no es muy bueno!"

Under those circumstances, you might be wondering about the purpose of this seemingly senseless, endless rant. Quite simply, I'm trying to drill one simple concept into your brain. So that it goes deep, deep, deep into your subconscious mind.

Certified Diamonds Don't Exist: (they just don't)

 Generally speaking, the following phenomenon are little more than coping mechanisms:

  • Big Foot.
  • Certified Diamonds.
  • Chupacabra.
  • Space Aliens.

According to an article in Psychology Today, people believe in conspiracy theories for the following reasons:

  • The desire for understanding and certainty.
  • A need for control and security.
  • An urge to maintain a positive self-image.

Given these points, it's easy to see why people want to believe in certified diamonds. After all, everybody buying a diamond seeks understanding and certainty.  Am I right?

At the same time, we all share a desire for control and security. In addition, it's reasonable to assume that you would like to maintain a positive self image. Which might explain the willingness of so many people to accept diamond grading reports as certificates.

The False Promise of Diamond Certificates:

In the final analysis it seems clear that anybody trying to sell you a "certified diamond" is lying to your face. Either that, or they might be marketing to your expectations (or so I've been told).

Diamond Grading Certificates.

Either way, it doesn't matter because the diamond grading reports issued by the AGS and GIA are not certificates. In fact, you'll find a bunch of legal jargon and mumbo jumbo printed on the back of the report that states this clearly.

The diamond grading report issued by a gemological laboratory states the diamonds’ characteristics at the time it was submitted for grading. That's it. Nothing more. Don't be misled into thinking that it's anything else.

With this in mind, the characteristics of a lab graded diamond can be good, or bad. The presence of a lab report merely reflects the sellers’ willingness to pay for it to be issued.

A gemological laboratory will issue a report on a chunk of frozen spit if somebody is willing to pay for it. Be that as it may, most people don't care as long as the jeweler shows them a certificate.

The Secret to Buying Lab Graded Diamonds:

"As you might've heard, it's always the small pieces that make the big picture." #DiamondTips #DailyQuotes #PuzzlePieces

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A basic understanding of the common 4C’s (Cut, Color, Clarity and Carat weight) is an absolute must. For that reason, you'll want to read this article on diamond clarity characteristics.  Which will help you avoid some costly diamond buying mistakes!

But for the moment, you would be wise to realize there is more to the overall cut grade than meets the eye.  For example, the spectrum for proportions ratings is too broad. Which is why so many AGS Ideal and GIA Excellent cut diamonds fail our performance standards.

In addition, many diamond grading reports failed to provide complete proportions data. Thus you are missing several pieces of the puzzle, especially if you're buying a fancy shape diamond.

To complete the picture, you need to determine the following:

  1. 1
    Table Diameter.
  2. 2
    Total Depth.
  3. 3
    Crown Angle.
  4. 4
    Pavilion Angle.
  5. 5
    Girdle thickness.
  6. 6
    Culet size.

But what do all those weird terms mean? What is a crown angle? And what does a diamonds' table or culet size have to do with anything? Let’s start at the beginning by learning what each part of a diamond is called and the rest will unfold as we go.

The Modern Round Brilliant Cut Diamond:

The modern round brilliant cut diamond has 58 facets separated into four basic sections:

  1. 1
    Crown Section (the upper half).
  2. 2
    The Girdle Edge (the line in the middle).
  3. 3
    Pavilion Section (the lower half).
  4. 4
    The Culet (the bottom point).

Crown Facet Diagram (RBCD):

Facet structure round brilliant cut diamond.

The top portion or upper half of the diamond is called the Crown. Which consists of a large octagonal facet known as the Table.

As you can see, the table facet is surrounded by eight triangular shaped Star Facets. The star facets point outward towards the girdle edge of the diamond. 

There are eight kite-shaped Bezel Facets radiate out from the table corners to the girdle edge. Which extended outward from sixteen triangular-shaped Upper Girdle Facets. Notice the arrangement of the crown facets and how the majority of them appear in pairs.

The Girdle is the edge of the stone that rests between the upper and lower portions of the diamond. Do not confuse this with  the girdle your grandmother wears to separate her proportions.

A properly cut girdle on a diamond resembles symmetrical ocean waves. Just imagine a symmetrical pattern of ocean waves pointing up and pointing down.

And no, we’re not going to further explore grandma's proportions. She’s already irritated enough. Yea, we did receive the flaming email pertaining to our inappropriate discussion of girdles. We care. Of course, we care. We’re just impertinent.  In case you're wondering, our Grammy taught us that word.

Pavilion Facet Structure (RBCD):

Pavilion facet structure round diamond.

The lower portion (bottom half) of a diamond is called the Pavilion. There's a small facet located on the bottom is called the Culet. The culet of a diamond is a tiny facet that finishes off what would otherwise be a sharp and brittle point.

There are eight elongated, kite-shaped facets called Pavilion Mains. These radiate up towards the girdles’ edge from the culet. Between each pavilion main facet is 1 of 16 elongated, triangular Lower Girdle Facets. This set of facets is arranged into eight pairs which redirect light towards the observer.

Hearts patterns are created by light reflecting off the pavilion main facets. The light from the pavilion mains reflects across the stone onto the lower girdle facets. Each time this happens it creates one half of a heart. The pattern of hearts is complete when light reflects all around the diamond perfectly. For more information, see the creation of hearts and arrows patterns.

How to Calculate Table Diameter:

Calculating Table Diameter RBCD

The table facet on a round brilliant cut diamond is the large flat facet on the top. As can be seen here with light blue arrows which highlight the edges.  Obviously, we put those there for your viewing pleasure. However, they also serve to indicate how to measure table diameter.

The expression “Table Diameter” or “Table Width” represents the size of the diamonds’ table facet in relative proportion to the overall size of the stone. It is usually expressed as a percentage of the girdle diameter, such as a 55% Table.

Which by the way is what is actually pictured here on a 1.023 carat stone. Don’t you just love the subtle accuracies of modern mathematical ray tracing software?

How to Calculate Table Percentage:

As stated previously, the blue arrows map out the path you will take to measure girdle diameter. The idea is to measure the distance across the table from corner-to-corner. Obviously, these lovely blue arrows will not exist to guide your way on actual diamonds.

Once you have taken these measurements, you will divide the longest table diameter by the average girdle diameter. Then you will multiply the quotient by one hundred.

Say what? Okay, in English… If the diamond measures 6.48 x 6.53 millimeters in diameter with a depth of 4.00 millimeters and its largest Table Diameter is 3.58 mm, then:

Average Diameter = (6.48 + 6.53) ÷ 2 = 6.505 mm, which is the average outside diameter.

Table Percentage = (3.58 table diameter divided by average outside diameter 6.505) = 0.5503458 × 100 = 55.03% which you would round off to a 55% table percentage. The table percentage is always rounded to the nearest whole percent.

How to Use a Micrometer or Table Gauge:

To begin with, you're going to have to get a jeweler to surrender their precious micrometer or GIA Table Gauge.

Be forewarned, this will be no easy task. As a matter of fact, you might have more luck plucking the lint from a plumber's crack. By the way, such a daring feat should only be undertaken while wearing rubber gloves!  In addition, sterile tweezers is a must! As I'm sure you realize, there's a pun in there somewhere.

GIA Table Gauge.

It's important to realize that jewelers tend to be a bit skittish. For this reason, they're likely to shut down and stare blankly if they sense you're analytical. With this in mind, you might find it challenging to complete this exercise.

Of course, you can buy your own table gauge from the GIA for around 20 bucks. Which is something you'll need to do if you're taking the GIA Gemologist course. But, otherwise, it's much easier to rely upon the table diameter measurement provided on the diamond grading report.

What is a Good Table Size for a Diamond?

Generally speaking, most round brilliant cut diamonds have a table diameter between 55 and 65%.  At the same time, the range of table diameter is 52.4 to 57.5% for an AGS-0 Ideal cut diamond. Of course, you should know that the GIA rounds off table size to the nearest full percent.

You'll discover that a table facet which is too large tends to create an imbalance of scintillation. To demonstrate this just imagine the flat flash affect that overpowers light reflecting from a fish pond. Another problem with large table facets is that they reduce the dispersion of white light into color sparkle/fire.

Regardless of that fact, here's how to calculate the table percentage of a fancy shaped diamond. Simply divide the width of the table by the width of the stone. Then multiply the resulting quotient by one hundred.

In addition, you should know that the table diameters of fancy shaped diamonds are measured width wise. This means that you will measure the diamond across the table at the widest point.

 Pro Tip:

Table diameter is always rounded to the nearest whole percent by the GIA laboratory. While the AGS laboratory reports the average measurement to the third decimal point.

How to Calculate Total Depth:

Now that you know how to determine the table diameter, it's time to learn how to calculate total depth. You'll do this by dividing the depth by the average girdle diameter. In order to get the depth, you'll measure the diamond from the top to the bottom.

Which means you'll measure the diamond from the top of the table to the bottom of the culet. That's right, you're going to have to weasel that micrometer away from the jeweler again.

Once you've got the measurements, simply divide the depth by the average girdle diameter.

Don't worry, if this seems confusing. Simply calculate the average diameter using the formula above for table size. Then divide the measurement from the top to the bottom of the stone by that number.

It's important to realize that the AGS proportion scale does not indicate a range for total depth. However, we prefer a total depth between 59-61.8%. In the event that all the other measurements are spot-on were willing to go up to 62%. But only if the crown and pavilion angle measurements and the degree of optical precision are tight.

How to Estimate Crown Angle:

The Crown Angle is determined by viewing the diamond from a side profile. Then visually estimating the angle of the dangle from the edge of the table down to the girdle edge.

Consequently, we openly admit that this method of visual estimation seems rather unscientific. At the same time, this is how it's always been done. As a matter of fact, this is how we did it at the beginning of my career. Remarkably with practice, most diamond graders get to where they're accurate within a percent or two. Pause for dramatic effect.

Instructions for Estimating Crown Angle:

Estimating Crown Angle.

Gently set the diamond in a pair of locking tweezers so that it's held by the table facet and culet. Hence, you'll be able to view the diamond clearly from the side profile.

Then place a flat object against the tweezers tips.  From this perspective, you'll estimate the angle of the crown as it relates to the edge of the pointer.  As an illustration, refer to the photograph on the left. In this instance, the diamond has a crown angle of 32 degrees.

In light of the potential for this scientific method to go awry, we verified the estimate using Sarin. Needless to say, we prefer the scientific method of measuring diamonds using computerized proportions analysis.

Be that as it may, a crown angle of 32° drops the proportions rating of this diamond down to AGS-3 Good. By the way, this is not the diamond that we used to create the diagrams used above.

 Pro Tip:

Be very careful when setting the diamond in the tweezers. It is very easy to chip the culet if the diamond is handled improperly! Be certain not to apply very much pressure.

Estimating Crown Angle:

Crown Angle Reference Chart.

As can be seen by the diagram on the left, estimating crown angle is pretty straightforward. For the most part, it's like taking a drafting triangle and selecting the corresponding angle.

In the first place, let's start by stating the obvious, which is that a right angle is 90 degrees. Correspondingly, this means that half of a right angle is 45 degrees.  Consequently, one-third of a right angle is 30 degrees. Obviously, this is not rocket science, but it's worthy of review.

In the same fashion, you can determine other crown angle measurements using these reference points. For example, a 25° crown angle is slightly shallower than 30 degrees. In like manner, a 34° crown angle is slightly steeper than 30 degrees. If this doesn’t make you appreciate our Sarin machine, we don’t know what will.

Average Crown Angle Measurements:

Given that most diamonds have a crown angle between 30 and 36 degrees there is a lot of wiggle room. In the event that the crown angle is shallower than 30° the diamond may look flat and lack luster.  At the same time, there may also be a noticeable loss of fire. Interestingly enough, the stone may still appear to be quite brilliant. Which means that it may exhibit a lot of brilliance (white sparkle) but very little fire (colored sparkle).

As a rule, we prefer a crown angle between 34.3 and 34.8° for round diamonds. Obviously, a crown angle of 34.5° is right on the money. However, it's also pretty difficult to find and is not likely to alter your perception of sparkle factor.

Consequently, a crown angle steeper than 35.3° tends to create more dispersion. On the other hand, it tends to do so at the expense of brilliance. Refer to the article 15 Seconds to Diamond Buying Success for more information

How to Find Crown Angle (RBCD):

Here is the proportions diagram for this 1.435 carat, E-color, VVS-2 clarity, Black by Brian Gavin diamond.  We use the colors blue and green below to highlight the crown measurements. This will make it easier for you to find the data that you need.

Crown angle proportions diagram.

The blue arrow on the left indicates that this diamond has a crown angle of 34.8 degrees. The crown angle of this diamond is within my preferred range of 34.3 and 35.0 degrees. Therefore, it should produce a virtual balance of brilliance (white sparkle) and dispersion (colored sparkle/fire).

The crown height measurement is indicated by the green arrows on the right. By the way, the crown height of 15.3% coincides with the crown angle of 34.8 degrees. Which is something you want to be aware of because sometimes the cutters hide extra carat weight in the crown section. Coincidentally, they're able to do this and still hit the right crown angles by adjusting the upper girdle facets

Computerized Diamond Proportions Analysis:

Consequently, if the little engineer-side of your brain has been screaming:

“That’s the stupidest, most antiquated system of measurement that I’ve ever seen! There should be a machine for that!”

Don't worry, we’re right there with you.

Sarin Diamension machine

In case you haven't figured it out yet, this is why we bought a Sarin DiaMension. Let's face facts, state-of-the-art computerized proportions analysis beats visual estimation every time. Plus, we just happen to be data freaks!

There's just something about being able to press a button. Oh, right! And get all the proportions details you need in a matter of seconds.  P.S. I love pressing buttons!

By the way, both the GIA and AGS use Sarin technology to measure the diamonds they grade. It stands to reason that if it’s good enough for them, then it’s good enough for us.

Improved Speed and Accuracy:

As much as the Sarin Diamension is accurate, it's also a sensitive piece of equipment. If there's the slightest speck of dust or lint on the stone, it feels the need to point it out.

Notably, it will say something brilliant like "dirt on facet seven."

Like we’re really going to be able to figure out which Frak'n’ facet is number seven!

Hal 2001 Space Odyssey

As a result, we find ourselves going back to the steam cleaner until every facet is sparkling clean. To be sure, something as innocuous as a fingerprint is enough to send our favorite toy into a tizzy.

Just imagine Sarin saying, "I'm sorry Todd, I'm afraid I can't work in these unsanitary conditions!" 

You'd think we were working with Hal from the movie 2001 - A Space Odyssey.

For the purpose of this reference, you may decide whether to think of the original or the sequel of the sequel.

As a result of these shenanigans, you can hear the voices singing the Sarin Song from the grading room:

"Bleep! Bleep! What the Bleep! Dirt on facet four my @ss!!! I’ll give you dirt on facet four!!!”

Which is always followed by the stomping of little footsteps as they wander off towards the cleaning unit (again).

Sarin is the Industry Standard:

Ah yes, the brilliant sound of perfection! You’ll be happy to know that some of the vendors we work with also use Sarin technology. Which is great because you're probably not chomping at the bit to buy one.

After all, it's not like you can use it to surf the Internet. Nor, will it print out nifty little 3D models that you can play with at your desk. Speaking of which, our friend Brian Gavin has one of those also!

Nevertheless, at least now you know how to visually estimate the proportions of a diamond the prehistoric way.

Fortunately for you, the AGS and GIA provide all the measurements on the reports for round brilliant cut diamonds. While the AGS Laboratory provides crown/pavilion angle measurements for fancy shape diamonds also, the GIA does not.

Be sure to read AGS Ideal-0 versus GIA Excellent cut diamonds. It will explain how the gemological laboratories measure diamond  proportions differently.

Measuring Girdle Thickness:

Remember, the girdle is the edge of the stone located in the middle of the diamond. It is the line between the crown (top half) and the pavilion (lower half). It is the edge of the stone that the prongs wrap around to hold the diamond in place. So, it needs to be just thick enough to let the stone be set securely.

An extremely thin girdle edge (less than 0.6%) may chip easily or be damaged. If the girdle edge of a diamond is extremely thick, it can be unattractive and difficult to set.

The girdle edge of a diamond may be faceted, or not faceted. It may be polished, or unpolished, the latter of which is known as a bruted girdle.  For more information on girdle finish referred to this article on Diamond Clarity Characteristics.

Laser Inscribed Girdle:

 Laser inscribed diamond.

The girdle edge should appear to be a thin to medium straight line with symmetrical peaks and valleys. This is a photograph of the thin to medium, faceted girdle edge of a Brian Gavin Blue florescent diamondThis photograph was taken at 70x magnification to show the inscription which is why it looks a little grainy.

The girdle edge has been laser inscribed with the logo for the Brian Gavin Blue florescent diamond collection and the lab report number.

On poorly cut diamonds the girdle edge will not be straight.  In fact, the girdle edge might appear to be wavy, or have areas that range from extremely thin to extremely thick.

An extremely thick girdle often creates large, fuzzy, gray reflections in the diamond. It also has a greater tendency to accumulate dirt and grime, especially if it's bruted. An extremely thick girdle also has the tendency to make grandma appear lumpy in all the wrong places. At the same time, it will still clearly separate the upper and lower halves. Enjoy the correlation.

Calculating Pavilion Depth Percent:

A diamonds’ brilliance is largely determined by the pavilion angle. Unfortunately, it seems that most jewelers like to focus on pavilion depth percent. Which is the distance from the girdle plane to the culet expressed as a percentage of girdle diameter.

Diamond cutters focus on pavilion angle. Which is the angle between the pavilion mains and the girdle plane. The steeper the angle the greater the depth percentage.

The difference between Pavilion Angle and Pavilion Depth Percent is an important point to understand. Sarin and OGI proportions analysis machines measure the Crown and Pavilion Angle measurements. However, they estimate pavilion depth percent based on those angles and other measurements.

Estimating pavilion angle

Speaking of estimating pavilion depth percent, this is a picture of how to do it the old fashion way. Which is basically the same gemological technique used to estimate crown angle described above.

Remember that depth measurements are estimates based upon the actual angle measurements. Therefore, you should focus primarily on the angle measurements when selecting a diamond.

Which does not mean that you should pay no attention to the crown height and pavilion depth measurements. Because we can use those measurements to determine whether the cutter cheated the stone. You can read more about that concept here.

How to Find Pavilion Depth (RBCD):

Below you'll find the proportions diagram for this 1.435 carat, E-color, VVS-2 clarity, Black by Brian Gavin diamond. It will help you identify the pavilion measurements.

Diamond proportions pavilion depth.

The pavilion angle of 40.7° is indicated by the blue arrow. The pavilion angle of this diamond is within my preferred range which is between 40.6 and 40.9 degrees. As such, it should produce a high volume of light return.

The ASET and Ideal Scope images on the diamond details page serve to verify the light performance.

The pavilion depth measurement is indicated by the green arrows. The pavilion depth of 43.0% coincides with the pavilion angle of 40.7 degrees. Therefore, we know that Brian Gavin did not "cheat the stone" in an attempt to hide carat weight.

What is the Best Pavilion Depth?

Most “experts” agree that for round brilliant cut diamonds a pavilion depth between 43 – 44% is optimum. They'll tell you things like:"The percentage can vary slightly because the crown angle and table size also affect a diamonds’ brilliance."

Together with: "The accepted range for pavilion depth of a round ideal cut diamond is between 42.2% and 43.8%."

In terms of Pavilion Angle (the accurate measurement) we prefer that it be between 40.6 – 40.9 degrees. Remember to think of light return by degrees instead of percentages. At the same time, you should know that a 42.5 - 43.3% pavilion depth correspond with this range of pavilion angle.

As it so happens, a pavilion depth of 43.5% is "the critical tipping point" where light begins not to strike fully off the pavilion facets. Which means that a pavilion depth of 43.5% is not likely to produce a higher volume of light return. Remember that the GIA rounds the pavilion depth measurement off to the nearest half a percent.

The Pavilion Angle Deviation:

It is important to note that other pavilion angles might also be acceptable. A shallower pavilion angle such as 40.4 – 40.5 degrees may go well with a steeper crown angle.  At the same time, a steeper pavilion angle of 41.2 degrees might go well with a shallower crown angle.

Sheldon Cooper quote

As I write that last paragraph I feel obligated to mention that I'm only saying that in hopes of mitigating the odds of being assassinated at the next trade show. It also might reduce the amount of hate mail from angry jewelers. Most of whom do not seem to understand how light reflects through a diamond.

Remember that the words may and might are used interchangeably in popular speech when referring to possibility and probability. Which means that a shallow pavilion angle may compliment a steeper crown angle. However, the odds are that it might not. Which means that there is a high probability that it won't.

I'm totally channeling Sheldon Cooper from The Big Bang Theory right now.

Of course, you can use tools like the ASET and Ideal Scope to validate the theory.

How Does the Pavilion Affect Performance?

Imagine a round diamond that displays an unattractive white ring along the edge of the table facet. This is known as a fish eye effect, but it's really a reflection of the girdle edge.  This frequently occurs when the pavilion depth is between 35 - 38 percent. It's a good indication that a diamond has been poorly cut. Why else would the pavilion depth be so shallow?

A pavilion depth of 49 – 51% might cause the table area and star facets to be dark and lifeless. This effect is known as a nail head. It's also an indication that a diamond has been poorly cut because it's too deep.

Although pavilion depth percentages can be measured and calculated, it is usually done visually.  Began by placing a pointer on the culet of the diamond. Then judge the distance of the reflection between the culet and the corner of the table facet. You will be viewing the diamond in the face-up position to obtain this measurement.

Estimating pavilion depth takes a lot of practice and is usually best left to the experts. Once again, this is where Sarin comes to the rescue and takes the guesswork out of the equation.

Diamond Culet Size:

The culet is the bottom point of a round brilliant cut diamond. It is the last step of the cutting process. Culet size is another factor that has traditionally been visually estimated. However, the Sarin measures culet size more accurately.

Visually Estimating Culet Size:

Imagine looking down through the flat facet on top of a diamond. From that perspective, you will be able to see all the facets traveling down to a central point. That's the culet which is the bottom point of the diamond. Interestingly enough, the bottom point of a diamond is actually a complete facet.

Diamond culet size medium.

Very small to large culet sizes can often be seen through the table without magnification. It looks like a small white circle and is often mistaken for an inclusion.

This 1.06 carat, J-color, VS-1 clarity, round brilliant cut diamond from Blue Nile has a medium size culet. The culet facet is visible through the table facet at this degree of magnification as indicated by the red hexagon outline on the photograph.

Our preference for culet size is “pointed” if you are considering a diamond graded by the AGS Laboratory. The same culet size will be “none” if the diamond is graded by the GIA Laboratory.

What Is a Pointed Culet?

For all intents and purposes, the terms none and pointed are synonymous.  In fact, the AGS Laboratory also used the term none for culet size until a few years ago. The Board of Directors determined that the term “pointed” was more accurate. That's  because technically there is always a culet facet. Which is why the AGS considers the term “none” to be inaccurate.

By the way, culet is pronounced Q-Let. The facet on the bottom of a diamond is not a steak cutlet. It’s a dead giveaway that you're a newbie if you refer to the culet as the cutlet.

Note that this diamond does not meet my selection criteria. It is strictly an example of a diamond with a medium size culet for demonstration purposes.

Grading Diamond Symmetry:

Symmetry for round diamonds is the difference between one that looks round like a properly formed circle, and one that looks like a flat tire. There is a picture for you to imagine, am I right?

Round diamond symmetry good.

This 1.00 carat, F-color, VS-1 clarity, round brilliant cut diamond has an overall cut grade of GIA Very Good. Which is kind of strange considering they graded the symmetry as only good. On the AGS grading scale the overall cut rate is based on the lowest denominator. That seems like a better system, don't you think?

This round brilliant cut diamond measures 6.11 - 6.27 x 4.04 mm. Which means that it is "out of round" as is apparent in this clarity photograph.

Understand that there are no perfectly round diamonds. However a round diamond with an outside diameter of 6.10 – 6.15 millimeters would appear more round.

 Tolerances for Outside Diameter:

Our preference is that the outside diameter of round diamonds be within 10/100th’s of a millimeter. Thus, an outside diameter of 6.55 – 6.65 is on the edge of being acceptable.  Which means that a round diamond measuring 6.55 – 6.66 millimeters is out of the question.

Obviously a smaller difference between the minimum and maximum measurements is preferable. However, don’t expect to find a diamond that is perfectly round. Remember that these puppies are turned on the wheel by hand. They're not stamped out like widgets on a production line, so there will always be variances.

Evaluating Symmetry in Fancy Shapes:

Heart shape diamond.

Symmetry in fancies is about ensuring that opposite sides of the diamond exhibit a complementary shape. In technical terms, symmetry is the equality between the corresponding sections of the diamond.

Take this 1.51 carat, E-color, VS-1 clarity, heart-shaped diamond from Blue Nile for example. Does the cleft on the left match the one on the right? Or is one side nicely rounded while the other is more straight and sharp?

I give you a hint. This is really nice looking heart-shaped diamond. Look at the outline. It looks like a really nice heart, right? There you go.

Factors of Diamond Symmetry:

Some factors of diamond symmetry are not as easy to detect as the outline of the shape. Generally speaking, these are the characteristics of symmetry you want to focus on:

  • Misaligned facets;
  • Facets which fail to point properly;
  • Misshapen facets;
  • Table/Culet facet misalignment;
  • Table facet not parallel to the girdle plane;
  • Wavy girdle edge (like a warped record).

In addition to these factors of symmetry, a fancy shape diamond might also have uneven corners and sides. The sides of emerald cut diamonds may have edges that are not parallel.

Be on the lookout for uneven wings on pears, marquises, and hearts. At the same time, be sure to look for uneven lobes on hearts. Also be aware of uneven shoulders on pear and oval shape diamonds

Last but not least, be aware that keel lines might be off-center in pear and heart shaped diamonds. It's important to realize that the pavilion facets of fancy shaped diamonds do not come to an end at the tip of the pavilion like they do on a round. In contrast, they form an edge known as the keel line.

What is Diamond Symmetry?

Diamond symmetry is largely a matter of semantics.

Some women will try to convince you that diamond symmetry looks like this:

  • A one carat diamond ring on her left hand;
  • A right-hand ring with a one carat diamond;
  • 2 carat total weight diamond earrings;
  • And a one carat diamond solitaire pendant.
Breakfast at Tiffany's

Which would make her perfectly symmetrical, or as she might say, "perfectly balanced".

By the way, if this scenario happens to describe the woman that you’re about to marry, then we’re about to become your new best friend. Look no further, you’ve found the perfect woman!

With regards to the symmetry grade of a diamond, we recommend a minimum of AGS Ideal or GIA Excellent. I wonder how many people reading that last sentence will catch the word minimum?

Because the diamond symmetry grade reflected on diamond grading reports refers to meet-point-symmetry. The gemological laboratories do not take optical precision into account as part of the grading process. Which is why we use ASET Scope, H&A scopes, and Ideal scopes to judge optical precision.

Girdle Outline (fancy shapes only):

Round brilliant cut diamonds are supposed to be round. So, it should not be necessary to judge the relative attractiveness of their shape or girdle outline. However, not all fancy shape diamonds are equally attractive or symmetrical.

As a matter of fact, the outlines of most fancy shape diamonds are simply not attractive. Which is not to say that some shapes of diamonds are not attractive. But rather, that the variations of the outlines might not be appealing. For instance, some oval shape diamonds look like ovals. While other ovals look more rectangular with blocky rounded corners which is more like a cushion cut.

Oval diamond symmetry.

Take this 1.00 carat, F-color, VS-2 clarity, oval brilliant cut diamond from James Allen for example. The diamond measures 7.51 x 5.61 x 3.56 mm. The length-to-width ratio is 1.34:1.00 which looks a little short and squatty from my perspective.

What do you think about the overall shape of this oval brilliant cut diamond? Do you find the outline appealing? Or would you prefer an oval brilliant cut diamond which is a little longer in shape?

Also take note of how different "the four corners" of the diamond appear in this photograph. Do the four quadrants of this diamond look even to you?

In the event that you're in the market for an oval-shaped diamond be sure to read this article. Because it contains the proportions I recommend for oval cut diamonds. Plus, there are lots of photographs that will help you better understand the visual properties of this shape.

The Symmetry of Perception:

Obviously this portion of diamond symmetry grading is judged visually. It is also subjective and largely depends on your personal preferences. However, it seems that the heads of pears and ends of ovals should be pleasingly rounded. Which is another way of saying that they shouldn't look blocky or flat-topped.

At the same time, the shoulders on pears, ovals, and heart shape diamonds, should be gently rounded. The shoulders should not appear to be blocky, squashed, or flat-topped. The sides, or wings, of marquises, pears, ovals, and heart shape diamonds should curve in attractive arches. If they are too flat, they tend to make the diamond look top-heavy or wide bellied.

Length-to-Width Ratio:

This only applies to fancy shapes…

The appeal of a fancy shape diamond is also affected by its length-to-width ratio. Some length-to-width ratios are more visually and psychologically pleasing than others. These vary depending on a diamonds’ shape. However, standard head/prong sizes are as follows:

  • Marquises between 1.75 and 2.25 to 1.00
  • Ovals between 1.33 and 1.66 to 1.00
  • Emerald cuts between 1.50 and 1.75 to 1.00
  • Heart shapes look best as close to 1.00:1.00
  • Pears shapes between 1.50 and 1.75 to 1.00
  • Rectangular cushions between 1.50 and 1.75 to 1.00

You might end up paying for the fabrication of a custom head if the diamond doesn't fit the sizes.

Calculating the length-to-width ratio of a fancy shape diamond is relatively easy. Simply divide the length by the width. Example: if a diamonds’ length = 7.53 mm and its’ width = 5.43 mm, then divide 7.53 by 5.43 = 1.39 to 1.00 or 1.39:1.00

Remember the Golden Rule:

First of all, try to remember that not everybody cares about diamond light performance as much as we do. Heck, the majority of the industry probably doesn't even understand the concept.

With that in mind, don't expect your local jeweler to be a walking, talking, diamond dictionary. They probably won't have the ability to spout mathematical prose at your every whim. Some will, but more than likely most won’t.

You may find this shocking, but even we have to look up stuff from time to time! It’s true. No, seriously, it’s true.

Whatever you do, don’t let this newly acquired information swell your head to dangerous proportions. There is no need to spark another fight on the gemological playground. Not every jeweler is geared up for the challenge. And you don’t want to get thrown out of another jewelry store, do you?

As has been noted (by certain members of the trade) there is nothing worse than an analytical. Or was that anal? To put it mildly, most diamond dealers don't have the patience to cope with our clients.

Truth be told, they're just not equipped to deal with over-excited, newly-educated, incredibly zealous diamond buyers. Especially one that doesn’t know better than to try to teach random jewelers how to calculate diamond proportions.

Bazinga! It's Totally Justified.

Justified Season 3 Cover.

Of course, a few of you naughty pranksters have shared your escapades with us. Your epic tales of running from jewelry store to jewelry store sprinkling happiness are delightful.

However, please stop dropping our name while on safari and collecting the heads of jewelers for your trophy wall. While you might just be passing through, we share the aisles with these people at the trade shows.

Last but not least, try to exercise restraint (a little okay, just a little) while on Safari. Lots of jewelers carry guns and they might feel justified shooting you if for no other reason than frustration.

Diversity Breeds Innovation:

Some jewelers sell diamonds based on a limited understanding of the 4C's. While a small number focus on cut quality and the light performance it creates.

It doesn't matter whether you’re a flower child who likes pretty things, or an analytical who likes data. We strongly suggest you find a jeweler who you identify with. You know, one that speaks your language. God help us, we speak Netite.

We hope you've enjoyed learning this "top-secret" industry insider information. This is one of the pages that got us sued in 1997 for disclosure of proprietary information to the public. Of course, we won, and this page has been updated a few times since then.

By this point, you should be able to tell whether a diamond is exceptionally cut. And you're probably better acquainted with the properties of frozen spit that you desire to be. But hey, that's actually a technical term used to describe diamonds lower than I-3 in clarity. It's also known as dead. Seriously, we're not kidding about that.

On a Final Note:

Keep in mind that a lot of this stuff is a matter of personal taste. Unless of course, you’re an engineer, software developer, architect, accountant, genius, or proctologist. In which case, you won’t be able to function, or sleep at night, until we find you a diamond.

Luckily for you, we specialize in finding super ideal cut puppies for our clients. Which is why we work with a select group of cutters who share our passion for excellence. What can we help you find?

Want us to give the diamond details page for a diamond that you're considering "the once over" using a professional set of eyes? Just ask by clicking the blue button above.

Ultimate Diamond Buying Cheatsheet free download

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