A Diamond’s Cut

Of the four “C’s”, nothing is more important than the cut.  No kidding, cut is the factor to pay the most attention to. Sorry to take all the romance out of this but a diamond is really just a house of mirrors, and if those mirrors are lined up just right the results are awesome.  Your diamond will explode with light and spectral colors!

So now you’re thinking “what’s the best cut?”  The short answer is there isn’t one. A lot of factors such as diameter to depth ratio, crown and pavilion angles, girdle angles, polish, symmetry, and on and on.  A better way to think of cut is light performance…the stunning combination of fire (rainbow colors), brilliance (white light reflection), and scintillation (patterns of dark and light facets sparkling)  You can spend days reading about it but when push comes to shove it comes down to looking at a diamond and seeing for yourself how it looks to you…this isn’t rocket science.  


Diamond Cut Chart

When reading a GIA diamond grading report the cut grade will be one of the following:     


Excellent – Very Good – Good – Fair – Poor

While that may seem self-explanatory let’s translate those terms into English. 

Excellent…the diamond is very bright.  It shows an even pattern with good contrast between light and dark areas, so the reflections appear crisp and well-balanced.  You look at the diamond and think “Wow”.

Very Good…most of the standards fall within the Excellent range with the exception of one or two factors.  A diamond with this cut grade will look very close to an Excellent grade.

Good… The Good cut grade diamond isn’t quite as bright – reflections aren’t as sharp and there’s more darkness or dullness in the diamond.  While the brilliance is still good, you look at it and think “that’s nice.

Fair… Fair Cut Diamonds offer much less brilliance, as light easily “leaks” through the bottom and sides of the diamond.   You look at this diamond and think “eh”.

Poor… Poor Cut Diamonds yield nearly no sparkle, brilliance or fire. Entering light “leaks” from the sides and bottom of the diamond.  You look at this diamond and think “is that thing a diamond?”


Other Factors Affecting Cut

As said before, since a diamond is a house of mirrors that depends on perfect placement, there are other factors that can affect how light returns to the eye.  Let’s look at a few.

Girdle Thickness

A diamond’s outermost edge is called the girdle.  They can be cut from extremely thin to extremely thick and everything in between.   The ideal girdle thickness should not be too thin such that it causes a heightened risk of chipping. On the other hand, the girdle thickness shouldn’t be too thick as it will result in “dead weight” which makes the diamond appear smaller.


Presence of a Culet

The culet is the sharp point at the bottom of a diamond.  Ideally, having no culet is best though a very small on is just fine.  As the culet is bigger it creates a bottom window for light to leak out of.



To put it simply, symmetry refers to how well aligned the diamond’s facets are.  Each facet should be exactly the same size and shape as the one opposite it. For all this to work properly, the diamond needs to be as round as possible, the table placed in the center, the girdle having consistency, and the top (crown) and bottom (pavilion) lined up perfectly.  Diamond cutting is all fun and games until you have to deal with these details…we kid you not.


Poorly Cut Diamonds


There a lot of poorly cut diamonds for sale out there which leads to the question…why would anyone buy them?  Many people simply don’t know better and don’t know the questions to ask. It’s also easy to be sucked into buying weight.  A poorly cut diamond may weigh 1.25 carats while the same diamond crystal cut properly may weigh 1.00 carat. Think of this diamond like a steak.  The perfectly cut 1.00 carat is the steak with all the fat trimmed off. With the 1.25 carat you’re paying the same price for the “fat” as you are the steak.  You may think bigger is better, but with poorly cut diamonds you’re wasting your money.


Evaluating Light Performance in Diamonds   

The most straightforward way to evaluate the light performance of a diamond is to observe it under different lighting conditions.  If you are trying to choose a diamond to buy, you can look at different stones and compare them on their brilliance, fire, sparkle, and contrast.

Alternatively, if you know the cut grades of several different stones, you can assume that the better-cut ones will have superior light performance compared with stones whose cut has a lower grade.  Of course, this is assuming that the diamonds do not differ dramatically on color or clarity.

Light performance can also be evaluated by using special equipment that measures how a stone reflects light and how much of it leaks out.  There are several devices that enable you to see exactly how light is reflecting or leaking out of a diamond. These include ASET (Angular Spectrum Evaluation Tool), Ideal Scope, and the Hearts and Arrows Viewer.




The cut is considered by many to be the most important "C." A poorly cut diamond, no matter how great its color and clarity is, will not compare in sparkle and "fire" to a better cut diamond with comparatively lower color and clarity.

There will always be different opinions on the best diameter of the largest facet on the top of the stone and the best depth for a diamond because these factors alone are not sufficient to accurately judge a diamond's cut. Other diamond factors such as crown angle, girdle thickness, pavilion depth-percentage, cutlet size, polish, and symmetry also play a role in judging a diamond's overall cut quality and should also be learned.


Each lab uses its own terminology which can sometimes be confusing. For example, GIA calls its top grading "Excellent" followed by “Very Good”, while other labs use the terminology “Very Good” as their top grade. A diamond graded “Very Good” by GSL, for example, would, therefore, be the equivalent of a diamond graded “Excellent” by GIA.

GIA: Excellent - Very Good - Good - Fair - Poor

HDR: Very Good - Good - Medium - Fair - Poor

AGS: Ideal - Excellent - Very Good - Good - Fair - Poor

DCLA: Excellent - Very Good - Good - Medium - Poor

GSL: Very Good - Good - Medium - Poor

While grades are not the same, it is typically recommended to buy a diamond with a “Good” grade or better regardless of who has provided a certificate. It’s not recommended to buy diamonds with grades of “Medium”, “Fair” or “Poor”, as the alignment of their facets may misdirect light so severely that it affects the brilliance of the diamond.



Proportions are not the only thing that affects cut. There are other factors that have to do with the type of geometry and quality of the end result.


In round diamonds, a thin to medium girdle is preferred (too thick will result in a smaller diameter for the respective weight; too thin has durability issues). This can vary with fancy shapes. For example shapes with sharp points are vulnerable to damage, so a thicker girdle ensures better durability. Side note: girdles can be faceted, polished, or bruted.


A culet is a pointed tip where the pavilion meets. It’s better to have no culet or a small one. Bigger culets can look like inclusions.


This refers to how consistent the diamond facets are. One misshapen facet can affect the facets surrounding it, setting off a chain reaction affecting angles, and reducing brilliance. Sometimes symmetry flaws are introduced on purpose in order to remove an inclusion and get a higher clarity grade. Poor symmetry affects the reflection of light so it's better to stick with “very good or excellent” symmetry for round diamonds. Fancy shapes should be evaluated on a case-by-case basis, but the symmetry grade can be "very good" and "good must be evaluated".

Common symmetry flaws include:

  • Off-center table or cutlet

  • Unequal facets

  • Misshapen facets

  • Wavy Girdle

  • Non-parallel table and girdle


Aside from a lower level of brilliance and light return, there are several telltale signs of a poorly cut diamond. Keep these in mind when shopping for a stone.



In longer fancy cut shapes like oval, marquise, or pear, there will often be black patches resembling a bowtie across the width of the diamond’s body. This occurs when the facets do not reflect light properly, and if it is present, you will see these black patches no matter which way you turn or tilt the diamond. Bowties occur due to light obstruction while viewing the stone, not light leakage. The actual process of “viewing” the diamond creates this effect—when an observer looks at a stone, the light rays traveling to the stone are literally blocked by the viewer’s head, which creates dark shadows that reflect within the stone. The cutter can possibly eliminate or lessen the intensity of bowties, but it may not actually be economically feasible to do so (losing too much weight = less profit).

On the bright side, slight bowties can actually give fancy shaped diamonds an appealing look that defines its overall character. The bowties that you want to avoid are the ones that blackout a large area. Bowties will almost always appear to some degree in fancy cuts like ovals, marquises, and pears, but the presence of a bowtie does not automatically render a stone worthless. If these stones were actually cut to completely eliminate any incidence of bow-ties, they’d generally be cut to dismal proportions and would have issues with brilliance. Just look for a stone where the bowtie blends in well.

Bowties are never mentioned in grading reports, and online diamond distributors can’t always provide you with visual assurance.


When diamonds are cut too shallow, the internal reflections of the girdle can be seen in the table fact, creating an image of a gray ring that is visually distracting. These stones suffer from a lack of brilliance, and the ring can sometimes resemble an I3 type inclusion. The severity of the fisheye can depend on factors such as:


  • Shallow pavilion angles — shallow depth can cause light rays to be reflected from the girdle back through the table. The light reflects in a way that causes less brilliance and makes pronounces cut defects

  • Large table facet — Larger tables reduce refraction

  • Girdle thickness — girdles leak light

Fisheye can occur in fancy shape diamonds as well! Shapes like ovals, pears, and hearts can exhibit this effect if they are cut with a bad combination of angles. The fish eye phenomenon is an indication of a very poor cut, which makes any diamond dull and lifeless.


If you have 3 GIA triple excellent diamonds, how can you tell which one has the best light performance? This is where tools like the ASET (Angular Spectrum Evaluation Tool) and IdealScope come in.

The ASET scope provides more information with regard to light performance. For fancy cut diamonds especially, the ASET is the preferred tool for evaluating light performance. However, it does take more knowledge to decypher what the colors mean and how they relate to the diamond's optics.

The simplicity of the IdealScope brings a certain degree of lucidity to the table. You do not really need any gemological qualification to figure out which aspects of an idealscope image to pay attention to. Red is desirable and indicates light return while diamonds with too many white areas indicate problems with light leakage.

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Diamond and Jewelry Buyer