Diamond Cut – Basics
If you take one thing away from us, make it what’s on this page.
15 minutes spent reading the information below could be the best time spent in all of your diamond research. Consider a bookmark. Unlike color and clarity, where high grades are rare, more than 60% of round brilliants receive the “Excellent” cut grade, despite having visible quality differences.
When shopping on the internet, it’s especially important to research the quality of cut, because this one “C” often has a visibly positive or negative influence on the other Cs, making it by far the most important.
Before going on: Check out the PriceScope Diamond Buying Guide
Diamond Cut Described
Diamond cut assesses craftsmanship and basic light behavior (for round diamonds). However, unlike the other Cs, laboratories disagree on how cut should be graded.
The Wild West of Cs
Unlike color and clarity, where the highest grades are rare, the majority of round brilliant diamonds receive the GIA’s top cut grade of “Excellent,” despite having visible differences in character and quality. Additionally, unlike color and clarity terms, the term “ideal cut” is widely abused, and has many different meanings and definitions, depending on the seller. Also unlike the other Cs, which are largely standardized, a diamond graded “Excellent” by one laboratory could be judged “Very Good” by another and “Fair” by still another. Most labs issue no cut grade for fancy shapes.
All diamond shapes are graded for polish, which analyzes how lustrous and unblemished the surface is – and symmetry, which judges the evenness and consistency of a diamond’s surface outline and facets.
Round Brilliant diamonds also receive an overall diamond cut grade – a conclusion by that laboratory about how bright or dark the diamond is. Diamonds cut with geometries too deep or too shallow may appear dark for various reasons, while diamonds with proportionate geometries successfully reflect, refract and return light to the viewer as brightness, fire, sparkle and contrast.
Be advised: Most diamonds are cut with deep angles, because it brings more profit to diamond producers.
Diamond cut: Let’s get practical
Warning: You can dive deeper and deeper into this topic – and that’s okay. You should at least break the surface and learn the limitations of grading reports. Beyond that, how far you choose to “nerd out” (and we mean that as a compliment) is up to you.
Not everyone needs to become a diamond cut rocket scientist, but if you want to be fully confident about the high quality of diamonds you’re considering, the methods below have been proven – again and again.
Choose the path that’s right for YOU
If you want to stay dry, just click back to the PriceScope Diamond Buying Guide and follow the shortcuts. But if you want to dive in and learn more about judging cut quality in one sitting than most diamond buyers will ever know, just keep reading. You might even become the person your friends all turn-to for diamond advice.
We’re not kidding.
Real world information
The inconvenient truth is that most diamonds are not cut well because carat weight has the most influence on price. That motivates diamond producers to keep the most possible weight in the diamond when cutting.
Diamond Cut: Planning
The typical rough diamond octahedron produces two finished diamonds. The primary stone has the most value. Diamond producers profit most by using angles as close to the rough outline as possible (circa 45 degrees) which can still earn a favorable grade like “Excellent” or “Ideal.” However, those angles often aren’t the best for brightness, resulting in darker, less lively diamonds.
Is that “Weight over Beauty?”
Yes. Most diamonds are produced for weight over beauty. This is possible because most laboratories maintain very loose diamond cut grading standards. As stated above, more than 60% of round diamonds passing through GIA receive their top cut grade of Excellent, despite being produced for weight over beauty (see statistics here). It’s not an exaggeration to say most diamond shoppers have probably never seen a diamond which was cut to have the best possible brightness and sparkle.
Bottom Line: Cut has the most influence on diamond beauty but is the least emphasized in many places, and the majority of diamonds are produced for weight over beauty.
Can I choose cut online with confidence?
YES. You may be happy to learn PriceScope is considered a real pain-in-the-neck by many diamond sellers.
This is because our community specializes in diamond cut assessment knowledge. We have more diamond enthusiasts with more experience over a wider range of diamond beauty than any other online resource. With a community made of of members who are consumers like you, you can be sure they have your best interests in mind.
PriceScope Round Diamond Subsets
The PriceScope community will help you predict the visual qualities of diamonds you are considering. Experienced members can identify discernible subsets or “makes” of round brilliant diamonds, three of which are described below (other subsets exist but are less frequent – we plan to detail them on a separate page).
|“Steep Deep”||“PriceScope Ideal”||“Super Ideal”|
The three examples below are hypothetically polished from the same starting rough crystal (the dark blue outline). As you can see they have different final weights.
The different final weights result from different cutting goals. Remember, the more cut attention a diamond is given, the more weight is polished away, and the smaller the finished diamond becomes.
The Steep Deep
In the world of round brilliant diamonds this subset is most abundant, by far. Producers use wide (steep deep) cutting angles to increase the diamond’s final weight, maximizing yield. Those wide angles cause some of the light entering the diamond to leak out of the bottom, instead of reflecting and returning to the viewer’s eyes. When removed from bright lights, steep deep diamonds quickly go dark and look physically smaller than they should.
Since they finish heavier than other subsets, steep deep diamonds are frequently less expensive for the relative carat weight. Using the illustrations above, this steep deep finished at 1.15 carats, notably heavier than the other two subsets would finish if polished from the same rough diamond crystal.
- Many steep deep diamonds have the GIA Excellent cut grade.
- Use the patented Holloway Cut Advisor (HCA) to identify and reject diamonds with light return issues.
- An Ideal Scope image will reveal light return issues in any round diamond.
The PriceScope Ideal
A PriceScope Ideal is cut with proportionate angles which successfully reflect and return light back to the viewer’s eyes as brightness, fire, contrast, and sparkle. Some specimens emphasize brightness over fire, or vice-versa, depending on specifics, but all have robust light return. A PriceScope Ideal scores Excellent on the Holloway Cut Advisor: When removed from bright lights it will remain brilliant and lively. Only 20% of round brilliant diamonds graded ‘Excellent’ or ‘Ideal’ meet our definition of PriceScope Ideal (PriceScope estimate).
Diamonds like this finish smaller than steep deep makes, making their performance proposition more expensive for the relative carat weight. Using the illustrations above, the PriceScope Ideal finished at 1.06 carats, notably lighter than the steep deep would finish, if polished from the same rough diamond crystal.
- Use the Holloway Cut Advisor (HCA) to identify diamonds with top light return.
- An Ideal Scope image can verify top light return in any round diamond.
- PriceScope Ideal diamonds earn the highest proportions or performance cut grade from any top-tier laboratory.
The Super Ideal
Only 3-5% of round brilliant diamonds graded ‘Excellent’ or ‘Ideal’ fall into the Super Ideal subset (PriceScope estimate), making them extremely rare. They are cut within a small range of scientifically proven “ideal” proportions which promote balanced brightness, fire, sparkle, and contrast. Beyond those basic proportions, their internal reflections are further fine-tuned to display 3D optical precision (aka Hearts & Arrows), proven in a specialized viewer.
The best Super Ideal diamonds are not casually or accidentally made. The narrow range of angles and fine tuning cause them to finish much smaller than steep deep diamonds, making their performance proposition decidedly more expensive for the relative carat weight. Using the illustrations above, the Super Ideal finished at 1.00 carat, which is significantly lighter than the 1.15 carat steep deep would have finished, if polished from the same rough diamond crystal.
- Super Ideals are frequently sent to the AGS for strict light performance grading
- An AGS Platinum report can confirm “Ideal” light performance
- Hearts & Arrows images (real, not computer generated) can confirm 3D optical precision
To identify Super Ideal diamonds graded by laboratories other than AGS, use the Holloway Cut Advisor (HCA) first, then ask for an Ideal Scope image to verify top light return. For any diamond sold as “Super Ideal,” actual Hearts & Arrows images should be provided.
The Premium Perception: True or False?
Does improved diamond performance come with a pricing premium? Or is there actually a discount for average cutting?
Remember these three examples have the same raw material cost. All three have the same color and clarity. All three earned the “Excellent” cut grade. On paper the only difference is carat-weight.
According to market cost (Feb 2021) check out these examples if they had H color, VS1 clarity and were all priced with the same retail markup:
To a casual shopper, the 1.06 and 1.00 probably appear to command “premiums” over the 1.15, but that’s not quite true. Remember, the 1.00 and 1.06 could also have finished at 1.15, but the cutter chose to keep polishing to improve brightness and optical quality. There was no premium added, instead more weight was removed to improve appearance. In that sense, instead of a premium for positive polishing, there is a discount for leaving it deep.
The “Deep” Discount
Steep deep diamonds are most abundant on the market because, lacking this education, casual shoppers assume they are getting more carat weight for the money. Most jewelry stores do not carry diamonds in the PriceScope Ideal or Super Ideal subset, so you may never have seen one in person before.
Our PriceScope Vendors
Successful online sellers like our recommended PriceScope vendors, have methods of reliably classifying and communicating the quality of a diamond’s cut, whether you’re examining it in-person or purchasing online. They additionally offer liberal inspection periods and free returns, to ensure that you’re delighted with your purchase.
- Start with diamond grading reports: Diamonds over 0.25ct should come with a full report (or certificate) issued by a gemological laboratory. Diamond color and clarity grading are subjective, so tolerances among labs vary – even between a single lab’s various locations – but the accepted standard deviation among reputable institutions is +/- one grade.
- Ask for photos and videos: First things first. Camera and monitor settings make diamonds look different screen to screen, so you can’t make nuanced decisions with images. With that said, videos and images are useful. And when a seller’s in-house images are consistently made, in one setup, then head-to-head comparisons within that seller’s inventory are practical.
- Look for performance scope and optical symmetry videos: The best proof of performance for any round brilliant diamond, apart from the AGS Platinum grading report, is actual ideal-scope or ASET photos. If the diamond is promoted as a Super Ideal diamond or Hearts and Arrows diamond, you should also expect hearts and arrows images.
- Ask the seller directly: If you have questions beyond what a diamond grading report and supplemental videos and images can communicate, pick up the phone, chat, or email and get in touch with the vendor. It’s in their best interest to communicate transparently with you. After all, they don’t want the expense of shipping a diamond, only to have it returned on their dime as well.
Get fast answers to any question: Ask our community of unbiased independent helpers.
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Intermediate Diamond Cut
If, instead of diamond looks and performance, we are trying to predict the looks and performance of an athlete:
- Average Proportions are like having the athlete’s weight and height.
- The Holloway Cut Advisor (HCA) outlines their physique for us.
- Performance Scope Images are like seeing photos of the athlete in action.
- Computer modeling is like having video proof of their abilities.
- Finally, in-person viewing with helpful input from a coach, is most decisive.
The physics of light-behavior within a diamond are reliably consistent, which means proportions measurements can be used to draw certain conclusions about the quality of a diamond’s cut and its likely appearance. There are eight facet groups on a round diamond: See Round Brilliant Diamond In Detail for a look at all of them.
You only need four primary proportions to take the next step and use the patented online Holloway Cut Advisor (HCA).
|Table %||Depth %||Crown Angle||Pavilion Angle|
With those four data points, along with the diamond’s carat weight and millimeter measurements, you are ready to use the HCA.
Two of those numbers are averages of eight measurements, and there is no way to know how far the minimum and maximum measurements are “off” from that average. In fact, with the exception of AGS Laboratories, nearly all data points on grading reports is averaged and additionally rounded.
|Major Proportions||AGS reports to:||Others report to:||Average of:|
|Table Facet||0.1%||nearest 1%||1 measurement|
|Crown Angle||0.1 degree||nearest 0.5 degrees||8 measurements|
|Pavilion Angle||0.1 degree||nearest 0.2 degrees||8 measurements|
|Girdle Thickness||0.1 %||nearest 0.5%||variable|
|Minor Proportions||AGS reports to:||Others report to:||Average of:|
|Star Length||1%||nearest 5%||8 measurements|
|Lower Half Length||1%||nearest 5%||16 measurements|
|Upper Half Length||Not reported||Not reported||16 measurements|
|Calculations||AGS reports to:||Others report to:||Average of:|
|Total Depth %||0.1%||nearest 5%||Not averaged|
|Crown Height %||0.1%||nearest 5%||Not averaged|
|Pavilion Depth %||0.1%||nearest 5%||Not averaged|
Proportions are terrific to use with broad predictive analysis tools. If you want to get into nuances like diamond cut consistency or 3D optical precision, we cover them below in performance scope images and computer modeling.
Holloway Cut Advisor (HCA)
The Holloway Cut Advisor (HCA) is a free tool that uses measurements of a round brilliant diamond from its grading report to calculate potential light return, fire, scintillation and spread.
The HCA is the best rejection tool available for assessing diamond cut. Its primary use is disqualifying proportions sets known to “leak light” and reduce brightness. While doing this it also gives reliable broad predictions about light return (brightness) and spread (physical size for carat weight), as well as hypothetical but logical conclusions about predicted fire and scintillation.
The HCA score is not diamond specific. It does not take the 40 minor facets into consideration (of 57/58 on the diamond). It does not account for nuances of cut consistency or 3D optical precision, for those seeking such detailed diamond cut analysis.
The HCA is the best overall prediction tool available. It simply but reliably predicts diamond brightness based on key data points. A comparison would be an “HCA radar gun:” Such a tool would reliably tell you, from a distance, what cars have unsafe drivers so you can reject going for a ride. It also identifies perfectly safe drivers, but is not capable of deducing which of those might be the “safest” driver.
PriceScope lists over 500,000 diamonds from the world’s top vendors in the only search engine which features the Holloway Cut Advisor as one of it’s filters.
Performance Scope Images
Online sellers focused on high quality diamond cut are increasingly adopting Ideal Scope and the AGS ASET (angular spectrum evaluation theory) scope. These are two reliable methods of proving or disproving that a diamond is properly returning the light it gathers from above to the viewers’ eyes.
- The Ideal Scope looks like a pink and black shot-glass.
- The ASET scope looks similar, subbing blue for black and adding green.
What the colors mean:
Positive Light Return
- Both scopes code light from the highest angles a dark color (BLACK or BLUE).
- The Ideal Scope codes all light entering from 0 to 75 degrees RED.
- The ASET scope codes light from 0 to 45 as GREEN and 45-75 as RED.
Negative Light Leakage (Windowing)
- The Ideal Scope uses a lighted stage for the diamond, so negative light leakage will be WHITE.
- The ASET scope typically uses a dark stage, so negative light leakage will be BLACK or GRAY (unless a lighted stage is used).
Superior: Diamonds cut with the best light return will show edge to edge red in both viewers. They will have symmetrical, evenly distributed contrast patterns seen as black (Ideal Scope) or blue (ASET). The only areas of leakage will be tiny symmetrical points of white at facet meet points and the girdle.
Average: Diamonds cut with average light return may have negative white/gray areas of leakage (windowing) near the center and/or at the girdle. They may show light drawn from disadvantageous low-angles (green) in ASET. Their contrast patterns, seen as black (Ideal Scope) or blue (ASET), may not be symmetrical or evenly distributed.
Poor: Diamonds cut with poor light return will have distinctive areas of negative white/grey leakage near the center, possibly showing as circular “rings” of leakage. They often show light drawn from disadvantageous low-angles (green) in ASET, particularly along the girdle/edge of the diamond. Their contrast patterns, seen as black (Ideal Scope) or blue (ASET), may only resolve partially and are asymmetric.
Ideal Scope and ASET Strengths
Structured lighting environments such as Ideal Scope and ASET are the simplest but most effective way to “map” the light traveling through a diamond. Brightness, leakage, and contrast can be meaningfully assessed, especially in standardized setups.
Ideal Scope and ASET Shortfalls
Performance scopes images provide a motionless view of brightness, leakage, and contrast, but the benefits of diamond cut performance occur in motion, over a full range of tilt. Different setups may also influence how much or little leakage is seen.
When buying a diamond online Performance Scope Images are a tremendous help in understanding the overall quality of a diamond’s light behavior as it relates to critical components of brightness, leakage, and contrast.
Think of a diamond as a tiny geometric sculpture where every facet has a purpose. The light behavior, particularly how bright or dark that diamond will be, depends on the alignment and orientation of all facets in relation to each other.
Computer modeling diamond cut begins with a 3D scan created in a diamond scanning device. In this process the diamond is placed upside-down on a rotary platform, surrounded by lights. The lights turn on and the silhouette of the diamond is captured eight times as it rotates. An integrated camera measures the angles of that silhouette.
The 3D model is then imported into software designed to follow the light as it travels through the diamond, taking its total sculpture into account. This computer modeling approach is called “ray tracing.”
The AGS Light Performance Cut Grade
In 2005, the American Gem Society introduced a Light Performance Cut Grade system based on ray tracing. A diamond’s 3D scan is used to determine values for brightness, leakage, contrast, and dispersive potential. Those values determine the diamond’s performance grade on a scale of 0 (best) to 10 (worst). A computer generated image (CGI) of the diamond as it will be seen in the AGS ASET performance scope is also calculated and imprinted on AGS Platinum Reports.
This is the world’s only diamond-specific cut grading system to date. In other words, only AGS analyzes each specific diamond sculpture in its entirety, tracing 40,000 rays of light with computer software. Other labs use a handful of average measurements, correlate them with charts and pencil in a diamond cut grade.
Dual and Triple Light Maps
AGS offers “dual light map” and “triple light map” certificates, as seen below. In these cases an ASET image is rendered for the bottom of the diamond. The only purpose of that rendering is to provide an impression of 3D optical precision, also known as “Hearts and Arrows.” While the 3D scan computer generated images can be very representative, a diamond’s optical precision is best judged with actual photos or in-person viewing.
You can read more about optical precision on our Hearts and Arrows Diamonds page.
DiamCalc is a system for polished gemstone modeling and design. The software can be used to generate a variety of photorealistic diamond images and videos, including performance scope (i.e. Ideal Scope, ASET) and optical precision scope (i.e. Hearts and Arrows) simulations. Professional versions of DiamCalc feature a ray tracing engine which may be used to design theoretical diamonds, in the same manner as architects use CAD to design and assess buildings prior to construction.
Images rendered from manual input (not useful)
Computer-generated images (CGI) can be produced by manually entering numbers from a grading report into the software. When this methodology is not disclosed it’s misleading. When broad average numbers are entered the software simulates perfect 3D optical precision for the missing facets (which doesn’t exist in nature) and produces images based on a theoretically perfect, unrealistic diamond wireframe.
Note: Manual input of grading report averages can be useful for broad conclusions regarding overall light return versus light leakage. Such use should be clearly disclosed as a non diamond-specific estimate.
Images rendered from a 3D Scan (useful)
Diamond 3D scans are useful for light performance assessment with ray tracing, as well as producing diamond-specific Ideal Scope and ASET CGI which are representative. Since all facets have been scanned in three dimensions the projections of light-behavior within the diamond are accurate.
Hearts and Arrows Analysis: Hearts and arrows images generated with a 3D scan may be very close to the diamond’s actual hearts and arrows but are not as decisive as actual photos or in-person viewing. You can read more about optical precision on our Hearts and Arrows Diamonds page.
Computer Modeling Summary
Because computer generated images have the appearance of actual photos, it is important to verify with the seller if the imagery they are providing is actual photography of the diamond, or CGI. If the images are CGI, verify that the seller secured a mechanical 3D scan of the actual diamond.
Computer Modeling Strengths
Computer modeling is the most sophisticated method of clinical diamond cut assessment. When imported, 3D diamond scans can reliably and accurately be used to place diamond cut in a performance category such as GIA Excellent or AGS Ideal, and to identify shape subsets such as Super Ideal, PriceScope Ideal and Steep-Deep.
Computer Modeling Shortfalls
3D scans of gemstones are created without any contact. The diamond is placed on a rotary platform surrounded by lights. The lights turn on and the diamond’s silhouette is captured eight times as it rotates. An integrated camera measures the angles of that silhouette. Device makers acknowledge potential linear error of ± 10 microns and angular error of ± 0.1 degrees per measurement.
With a high-quality 3D scan, much can be determined about overall diamond cut performance. For extremely nuanced analysis regarding matters like optical precision, or hearts and arrows, it’s best to use actual images or view the diamond in an optical precision scope in person.
Diamonds are traditionally shown to prospective buyers in a single location, usually under spotlights which make all diamonds look good.
It’s important to remember that diamond cut is all about how a diamond looks as it travels through the world’s infinite panoramas of illumination. Diamond beauty is dependent on the specific diamond, the specific environment, the specific viewer and – most of all – the light environment.
Ensure Environmental Variety
If the three “Ls” of real estate are “Location, Location, Location”, the three “Ls” of cut are Lighting, Lighting, Lighting.
Instructions for Thorough In-Person Viewing
A. Setting Up The Diamonds
- If comparing 1 diamond, use tweezers or a temporary mounting
- If comparing 2-3 diamonds, place them in temporary mountings
- If comparing more than 3 place them side by side on a white diamond-viewing tray
- Be prepared to return, with your favorite two in temporary mountings or on your skin
B. Lighting Definitions
- Spotlighting: Small intense lights overhead. These are seen in every jewelry showroom and most household bathrooms (facilitates brightness-dispersion-scintillation)
- Natural Lighting: Large window or porch area where sunlight/daylight is visible (facilitates brightness-dispersion)
- Diffused Lighting: Fluorescent tubes or diffused indirect light, as in a school classroom or office (facilitates brightness)
- Low Lighting: Darkened room, corner, or other areas where no overhead light exists (reveals leakage)
- Filtered Lighting: Outside area in sunlight under a tree with leaves moving (facilitates scintillation)
C. Prepare Yourself and Other Viewers
- Is someone wearing eyeglasses? They will perceive dispersion and scintillation differently.
- Are there observers over 40? Presbyopia has often set-in, so go ahead and keep the reading glasses on if they want to get close.
- Are there observers under 21? They may detect nuances others do not, as they can focus as close-distance.
- Is someone wearing a yellow or brown shirt? Diamonds reflect their surroundings. They will take on that color appearance in general and in their performance qualities.
Take the tour!
Stop 1: Spotlighting:
Start under overhead spotlights as you would see the diamonds in a jewelry store. Compare the diamonds at 18 inches. Hold them still, then move them slowly side to side. Then move them rapidly. Look for overall brightness (white and colored light). Ask which has more white sparkle (isolate glare), which has more colored sparkle, which seems more vivid (more intensity in brightness and darker contrast). Does one have more rapid/small sparkle while another has more broad/large sparkle?
Now move them from arm’s length slowly to very close up. Does one get darker as it comes closer? Do they both remain as-bright? Do they both remain as vivid in brightness and contrast? If you’re inclined to take notes, by all means do, since these comparisons will be made in a few different lighting conditions.
Stop 2: Natural Daylight
Whether outside or near a window, stand with daylight at your back so the diamonds are illuminated by natural, completely outdoor lighting. Go through the same comparative exercises listed above. If it’s a cloudy day you can expect to see far less fire. If it’s a sunny day the diamonds may actually look somewhat dark inside, but the colored flashes should be electric.
Stop 3: Diffused Lighting
If you don’t have specific classroom or office-style lighting you can diffuse overhead lights with a pillowcase or sheet. There should be no spotlights or direct lighting of any kind in this environment. These conditions resemble the diffused northern daylight of a diamond bourse. Much like a cloudy day, you will see little or no fire here. Go through all of the same comparisons, this time focusing on how bright the comparative diamonds remain, and how symmetrical the contest patterns are.
Stop 4, Low Lighting:
This can be done under your desk or kitchen counter. No direct light at all is preferred. In a pinch you can cup one hand over the other that is holding the diamond and see how it appears when gathering only the most limited light. A very important test in this condition is to see if either diamond “gets smaller” than the other, or if they both retain their side-to-side size appearance.
A diamond that seems to “get smaller” may not be returning light from edge-to-edge as the finest cut diamonds will – and one where you literally see through the center is leaking light through the pavilion.
Stop 5, Filtered Lighting:
Many diamond professionals have discovered the joy of viewing a well-cut diamond on a bright day, but standing underneath a tree where the leaves are moving and permitting sunlight to filter down to the diamond.
Why is this such a great environment? Ambient sunlight reaches down to the horizon, promoting edge to edge brightness, while the leaves break up the cascading sunlight, creating abundant scintillation events. Meanwhile, your pupils have contracted (because your brain registers the sunlight) and in that state they clip abundant dispersive fans, causing your optic nerve to perceive far more colored flashes than you do in other illumination scenarios.
It should go without saying but be sure the diamond is completely secure and will not fall or pop out of the tweezers if you choose to take it outside.
Get fast answers to any question: Ask our community of unbiased independent helpers.
Ready to find your diamond?