Of the 4Cs of diamonds – cut, color, clarity, and carat weight – the most important is cut.
This is because a diamond’s cut has the greatest impact on its beauty. The better its cut, the greater the diamond’s brilliance, fire, sparkle, and overall “wow” factor.
A top quality diamond cut can make a stone look bigger than it is, hide internal flaws (known as inclusions), and reduce the appearance of any off-white color tint. On the other hand, a badly cut diamond looks awful: dull, discolored, included, and small for its carat weight.
Diamond cut is an incredibly complicated topic – gemologists and mathematicians have been studying and debating it for decades – and there’s much more to this rabbit hole than can be covered in a single article.
But if you’re here because you’re a normal person who just wants to buy a beautifully cut diamond, instead of getting stuck with one that’s not as great as marketing tactics would have you believe? Then this guide is for you.
Quick Navigation
What is Diamond Cut?
Because the term diamond cut is commonly used to mean several different things, let’s first get some quick definitions out of the way.
Diamond Cut as in Cutting Style
A diamond cut’s can sometimes refer to the particular diamond cutting style that was used to shape and facet it. In this sense, diamond cuts generally fall into one of two styles: brilliant cut and step cut diamonds.
As its name suggests, a brilliant cut diamond is crafted to maximize light reflection and refraction. Cut into numerous facets of various shapes, brilliant diamonds amplify sparkle and fire to their fullest extent.
Diamond shapes that are cut in a brilliant style include round cut, princess cut, radiant cut, and cushion cut diamonds.
A step cut diamond is all about clean lines and geometric precision, and is cut in a square or rectangular shape. Facets arranged parallel to the diamond’s edges create a “staircase” effect leading up to a large, flat center facet that showcases the stone’s clarity and shine.
Popular step cut diamond shapes include Asscher cut, baguette cut, and emerald cut diamonds.
Diamond Cut as a Synonym for Diamond Shape
Often when talking about diamond cut, people are actually referring to diamond shape – that is, the geometric shape a diamond is cut into (round, oval, pear-shaped, and so on). This usage is technically incorrect. But it’s understandable why it’s so commonplace, since most diamond shapes have “cut” in their names.
Diamond shapes are classified into two major types: round brilliant cut diamonds and fancy shapes.
The round brilliant cut diamond gets its own category because it’s by far the most popular diamond shape, especially for engagement rings.
Any diamond that’s not a round brilliant is known as a fancy cut or fancy shape diamond.
See also: The Top 12 Most Popular Diamond Shapes
Diamond Cut in the Context of the 4Cs
Then there’s the definition of diamond cut that’s used when talking about the 4Cs of diamonds. In this context, diamond cut refers to the cut quality of white round brilliant diamonds: their proportions, symmetry, polish, and overall appearance.
As this guide is about diamond cut as it relates to the 4Cs, this is the meaning we’ll be using from here on out.
According to the Gemological Institute of America (GIA) – which is the world’s leading and most trusted impartial appraiser of gemstones – round brilliant diamond cuts can fall into one of five different quality grades:
- Excellent (the best)
- Very Good (not bad)
- Good (mediocre)
- Fair (bad)
- Poor (awful)
(Note that fancy shaped and fancy colored diamonds don’t receive cut grades, as they are evaluated according to different sets of criteria.)
Of these five grades, Excellent cuts provide the best light performance. That is, the diamond’s ability to interact with light to produce the following aesthetic effects:
- Brilliance: The reflection and contrast of white light in a diamond.
- Fire: A diamond’s dispersion of white light into flashes of rainbow colors.
- Scintillation (commonly called sparkle): The flashes of light and dark areas within a diamond as it or a light source moves.
Needless to say, you should always choose an Excellent cut for a diamond engagement ring!
However, there’s more to truly top-tier diamond cut than just its assigned cut grade. Because cut is so important to a diamond’s appearance, you want to make sure you select an Excellent cut diamond that truly lives up to its name.
In the following sections, you’ll learn the step-by-step process for doing just that.
Step 1: Create a Shortlist
The best place to begin your diamond shopping journey is online. The reasons being:
- Online vendors have far wider selections of diamonds to choose from. Most physical retailers don’t have the capital required to keep a large inventory onsite, which severely limits your options when shopping at a brick-and-mortar store.
- Online retailers have lower overheads than physical stores, which can translate into significant savings on your purchase.
- Reputable online vendors offer high resolution images and magnified videos of their diamonds, giving you a closer view than what you’d get at a physical store.
- They also offer risk-free returns policies, negating the risk of buying a diamond you haven’t physically seen.
- You can browse and shop at your own pace, without being subjected to pushy sales tactics.
Some well-regarded online jewelry stores that offer these benefits include James Allen, White Flash, Brian Gavin, Ritani, and Blue Nile.
Visit any of these retailers (or others of your choosing) and create an initial shortlist of round diamond candidates. You’ll be able to refine your search results to Excellent cuts only, as well as set your chosen parameters for color, clarity, carat weight, and more.
Pro tip: We recommend shopping for loose stones instead of pre-set diamond engagement rings. A ring setting can mask flaws and color tints in a diamond; it’s better to view the stone from all angles before making a purchasing decision. Plus, with a loose diamond, you get to choose the exact setting you prefer.
Step 2: Verify the Cut Grading Scores
Once you’ve narrowed a selection of Excellent diamonds, the next step is to go through each stone on your shortlist and check its diamond report – for reference, a sample report from GIA can be found here.
Any reputable diamond vendor will provide a report for your inspection upon request. Many online vendors make an electronic version of the diamond report available directly on the diamond’s product page. If a vendor can’t or won’t produce one, that’s a major red flag that the diamond is probably of subpar quality.
You should only consider diamonds that come with GIA reports, as GIA’s gemstone laboratories provide the most trusted and high quality diamond assessments. While other labs also offer reports, their grading standards are generally less robust and reliable.
Under a GIA report’s Grading Results heading, you’ll see the diamond’s overall cut grade – Excellent, Very Good, Good, Fair, or Poor.
And under Additional Grading Information, you’ll see two separate grades for Polish (how smooth a diamond’s facets are) and Symmetry (how well its facets align), which are also scored from Excellent to Poor.
If all three grades are Excellent – in what’s known informally as a “Triple Excellent” diamond cut – you can move to the next step. If they aren’t, then discard that diamond from your shortlist.
A note on terminology:
Some sources and vendors may refer to top-graded cuts as Ideal instead of Excellent. Ideal was the top diamond cut grade awarded by the American Gem Society (AGS), which used to be the other premier provider of diamond assessments besides GIA.
AGS Laboratories no longer produces diamond reports itself, having merged operations with GIA at the end of 2022. However, you may still see AGS-graded diamonds in the wild, and some people still use Ideal to refer to the highest diamond cut grade.
If considering an AGS-graded diamond, then what you’re looking for is an AGS-000 or “Triple Zero Ideal” cut. Like GIA’s Triple Excellent, this simply means that the diamond has been awarded top marks for cut, polish, and symmetry.
Step 3: Check That the Diamond Has Ideal Proportions
It’s not enough to pick a Triple Excellent or Triple Zero Ideal diamond and call it a day, because even these diamond cuts mostly don’t have ideal proportions.
A diamond’s proportions are critical to its light performance – whether it will be a knockout stunner from across the room, or just “meh”. This is because round brilliant diamonds must be cut to precise mathematical standards in order to maximize their light performance. Deviating even slightly from these will rob a diamond of its full beauty.
For instance, a diamond that’s cut too deep relative to its width allows some of the light that enters the stone to escape through its sides, instead of being refracted and reflected back out of the top. Similarly, diamonds that are cut too shallow allow light to leak out through the bottom.
As mentioned, most diamonds don’t have ideal proportions. The reason for this is diamond carat weight is the primary driver of a diamond’s value, and stones cut to ideal proportions lose more weight in the cutting process than ones that aren’t.
In other words, there’s a strong economic incentive to produce diamonds cut with mediocre proportions, as they can be sold for greater profit.
So how do you tell if a diamond does have ideal proportions? It’s actually very simple to figure out – but before we get into the details, here are some quick round brilliant diamond anatomy definitions you’ll need to know.
Basic Round Diamond Anatomy
- Crown: The upper part of a diamond. It includes the flat center table facet and the ring of smaller facets around it.
- Pavilion: The lower cone-shaped part of a diamond. Though hidden when the diamond is mounted in a ring setting, the pavilion still plays a big role in light performance.
- Girdle: The circumference and widest part of a diamond. It separates the crown from the pavilion.
- Table: The uppermost and largest facet of a diamond.
- Culet: The lowermost facet or point cut on a diamond.
Knowing these terms is necessary to assess the proportions of a diamond which have the most impact on light performance. These are: length-to-width ratio, depth percentage, table size, crown angle, pavilion angle, girdle thickness, and culet size.
Now let’s go through each proportion, using the sample GIA diamond report as a guide.
Length-to-Width Ratio
A diamond’s length-to-width ratio describes the diamond’s face-up proportions. For round brilliant cut diamonds, the face-up shape should be a perfect circle, so the ideal length-to-width ratio is 1. A variation of up to 0.02 is acceptable.
To find the length-to-width ratio of a round diamond, simply divide its minimum diameter measurement by its maximum diameter measurement.
In the sample GIA report, the diamond’s measurements are listed as 6.41 – 6.43 x 3.97 mm (minimum diameter – maximum diameter x depth).
Using the first two of these measurements, we can calculate this diamond’s length-to-width ratio as follows:
6.41 ÷ 6.43 = 0.99
Being only 0.01 off from a perfect 1, this diamond qualifies as having an ideal length-to-width ratio. If it were a real diamond you were considering, you could keep it on your shortlist and move on to checking the next proportion.
Depth Percentage
The depth of a diamond refers to its height from the table to the culet. A diamond’s depth percentage is calculated by dividing its depth by its average diameter, and multiplying the result by 100.
You don’t need to calculate this yourself, as diamond reports already do it for you. For instance, in the sample GIA report, you can see the depth percentage of 61.8% marked directly on the Proportions diagram.
For a round diamond like this one, our recommended range for ideal depth percentage is between 59.5% to 62.5%. This is the sweet spot that allows for maximum light refraction and reflection.
Some sources give wider ranges. However, 59.5% to 62.5% produces the best light performance – and if you’re going to spend all that money on a diamond anyway, why not get the most spectacular and impressive diamond cut possible?
Table Size
A diamond’s uppermost facet is called the table. Being the largest facet of a round diamond, the table takes in and reflects back the most light.
However, when a diamond’s table is too large relative to its total diameter, the stone will lack fire. On the other hand, a table that’s too small will result in subdued brilliance.
For round brilliant diamonds, the ideal table size is between 54% to 57% of their total diameter. This range provides the optimal balance of both brilliance and fire.
In the sample report, the table size is marked as 58% on the diamond Proportions diagram.
You can see that for this criterion, the sample diamond doesn’t pass muster. If it were a real diamond on your shortlist, now would be the time to remove it.
It may seem pedantic or unnecessary to cull a diamond you like that’s “only” 1% out of range, for this or any other parameter. But the truth is, there are plenty of other diamond cuts out there that do correspond to ideal proportions. When you find yours, you’ll see just how beautifully it performs compared to the mediocre masses, and be glad that you made the effort!
Crown Angle
A diamond’s crown angle is formed from the intersection of the crown bezel facets and the girdle plane. For round brilliant diamonds, our recommended ideal crown angle is between 34° to 35°.
This is the range where a round diamond yields the most perfectly balanced return of both brilliance and fire, instead of sacrificing one for the other.
Referring again to the sample report, we can see the crown angle marked as 35.5° on the diamond diagram.
For this criterion as well, the sample diamond doesn’t correspond to ideal proportions.
Pavilion Angle
The pavilion angle is formed where the pavilion main facets intersect with the girdle plane. GIA reports show a diamond’s pavilion angle on the diamond diagram, underneath the crown angle.
For a diamond cut in a round brilliant shape, our recommended ideal pavilion angle is between 40.6° to 41.0°.
Steeper pavilion angles can diminish the diamond’s brightness, while shallower angles can contribute to an ugly optical effect called fish-eye (yes, the stone displays a pattern that looks like a dead fish’s eye – yuck).
Girdle Thickness
As the band that separates the crown of the diamond from the pavilion, the girdle functions to protect the stone from damage during the setting process.
A diamond’s girdle shouldn’t be too thin, as this increases the risk of chipping and breaking. But it also shouldn’t be too thick, which makes the diamond appear smaller by retaining unnecessary weight in the girdle.
Round diamond girdles don’t have uniform thicknesses. Therefore, girdle thickness is assigned within a range based on the thinnest point and the thickest point of the girdle.
That’s why in the sample report, the diamond’s girdle thickness is described as medium to slightly thick.
Our recommended ideal girdle thickness is between thin to slightly thick. This means a girdle can be thin to medium, medium to slightly thick, or thin to slightly thick. These are all acceptable girdle thicknesses.
Here’s some good news: a diamond cut can’t be graded as Excellent if it falls outside this range. And since by now you know you should only consider Excellent diamond cuts, checking for this ideal proportion takes care of itself!
Culet Size
The culet is the bottom point or facet on a diamond. It can be assigned one of eight sizes, beginning at none (where the culet is cut as a point rather than a separate facet) and finishing at extremely large (where the culet is 15% or more of the diamond’s diameter).
A large culet can be very noticeable to the naked eye, even to the point of resembling an unsightly inclusion under the diamond’s table. Large culets also lose more light through the bottom of the stone than pointed (none) ones.
Accordingly, our recommended ideal culet size is none (or pointed, for AGS-graded diamonds).
Recap: How to Pick the Best Diamond Cut
In summary, here are the criteria you need to check in order to find a terrific and super-sparkly diamond cut.
- Cut Grade Scores: Triple Excellent or Triple Zero Ideal (top marks for overall grade, polish grade, and symmetry grade)
- Length-to-Width Ratio: 1 (0.02 deviance acceptable)
- Depth Percentage: 59.5% to 62.5%
- Table Size: 54% to 57%
- Crown Angle: 34° to 35°
- Pavilion Angle: 40.6° to 41.0°
- Girdle Thickness: Thin to Slightly Thick
- Culet Size: None/Pointed
Thanks for reading and we hope you found this information helpful! If you’d like to learn about other aspects of diamond quality, then why not try our guide to diamond clarity next?