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Cut Grading <The Ultimate Diamond Guide>

*Do note that our writings are based on our personal opinion and only serves as a reference or guide.

In this article, we will explain how a cut grade is measured, and how its components proportions, polish, symmetry and light return affect the diamond's outlook.

Cut Grading

Diamond cut refers to how the Diamond facets are being cut which include the angles, the proportions, the symmetry, brilliance, fire, scintillation and finishing details of the diamond. These components directly impact how a diamond sparkles, which is what makes diamond so fascinating to our eyes.

A poor cut Diamond and an excellent cut diamond will look vastly different to our eyes even if all other 4 C’s stays constant. GIA grades cut on a scale of Excellent, Very Good, Good, Fair and Poor, separated into Cut, Symmetry and Polish. In our opinion, this is the most important factor of a diamond.

A diamond with Excellent Cut, Symmetry, Polish grading is what is known as a triple excellent diamond, which is sold at a premium.

There have been developments measuring Light Return using an ASET Scope. We will go through it later in this article.


The cut of the diamond measures the proportions in which a diamond is fashioned into. For round brilliant diamonds, a certain range of of percentages and angles would be deemed as an 'Excellent' diamond.

To Begin, here are the main proportions that are measured in grading a diamond.

1. Table size (%)

· Larger tables sizes will show more brightness, but being too large will reduce fire.

· Table is measured using a micro ruler and calculated as a percentage of the average diameter of the diamond.

2. Crown Angle/Height

· Shallow crown height/small grown angle creates a fish-eye effect (seen below)

· Crown angles are measured using a protractor

3. Pavilion Angle/Depth

· Deep pavilions make the center look dark, shallow ones create a nail-head effect (seen below)

· There are various ways to measure and estimate pavilion angle

4. Girdle Thickness

· Thick girdle protects the diamond from chipping, but being too thick adds unnecessary weight

· It is measured as a percentage against the diameter of the diamond

5. Cutlet Size

· Large cutlets are visible from the table face-up as a dark spot on the center.

· Some cutters add a cutlet to prevent chipping

6. Others: Star Facet (%), Lower Half (%)

· These are estimations that do not affect the appearance of the diamond as much as the above components

A diamond with good proportions will shine brilliantly as light will return from its table facet. If it is too narrow, it will have a fish-eye effect, too deep will have a nail-head effect.

Proportions Grading Chart

The full GIA Diamond Grading Certificate will provide all these information necessary, which you can check if it falls under the excellent category using the chart below. The GIA Diamond Dossier Certificate however will not provide all the necessary information which will make buying diamonds online a riskier, given that you cannot inspect the diamond in real life.

Ideal Cut Grade - Only in AGS Grading

As explained in our article comparing grading laboratories, AGS has an additional cut grade above excellent known as the 'Ideal Cut' which in their scale is better than 'Excellent'. This makes up of only 1% of the diamonds graded by AGS (not GIA), while 3% of diamonds graded by GIA are in the excellent cut range. These AGS Ideal are possibly in the red box drawn - a smaller range in the GIA excellent grade. Certain diamonds may be sent to AGS instead if it is suspected to be an 'Ideal'.

There are overlaps in awarding the cut grade, but we have included an approximate based on the data we have collected and our market knowledge. It is not definitive but it serves as a guide.

Also be very wary of marketing terms used. Some retailers may calls their diamond Super Ideal, Perfect Ideal etc., but it is important to do your due diligence and check the official grading on the AGS certificate, rather than rely on in-house printed certificates.

Our Recommendations

In our opinion, it is very important for a diamond to have an excellent cut (proportions) grading. Even so, a diamond with GIA excellent cut will only ensure you have a decent looking diamond, but not necessarily the best. Hence, if the diamond does not even quality as a GIA excellent, it would lack in the overall beauty and be quite visibly dull.

An AGS Ideal would be a good way to ensure you have the better diamond out of the GIA excellent range. However they are sold at a premium. Hence knowing how cut (proportions) is graded can help you pick out diamonds that can qualify as AGS Ideal diamonds even though it only comes in the GIA certificate. As mentioned, this can only be done with the full GIA Diamond Grading Report.


Symmetry measures how identical the facets are and if the diamond facets are aligned with one another. Like polish and cut (proportions), it is graded from: Excellent, Very Good, Good, Fair to Poor.

Common Type of Symmetry Problems

  • Misalignment [1], Misshapen Facet, Non-pointing [2], Not-octagonal Table

  • Out of round, Table or Cutlet off-center, Table-cutlet not-aligned

  • Girdle Thickness Variation [3], Crown or Pavilion; Height or Angle Variation

  • Lower or Upper Half Variation [4]

  • Missing Facet, Extra Facet

How can we check for symmetry using a Hearts and Arrow Scope?

A Hearts and Arrow Scope is common used in many retail stores. It is a quick tool to show facet arrangement and allows customers to make a quick judge of symmetry.

On the top row, it shows the hearts seen from the pavilion view. A diamond with an excellent symmetry will have even and symmetrical hearts. Same goes for arrows, seen from the face-up view, should be uniform and complete.

Actual View of a Hearts and Arrow Scope

Upon using an actual Hearts and Arrow Scope may not be that easy as the view is a lot smaller than these images. Here are the key things to look out for and scrutinize when using the scope. The left diamond shows a perfect symmetry example while the right is one of an imperfect diamond.

There should be 8 regular shaped hearts and arrows of identical intensity.


· A1: Arrows well-formed, distinct v-shape

· A2: Equal gap between hearts and arrows

· A3: V-points on heart aligned with arrows


· F1: Uneven arrow length

· F2: Unequal gap; thin and thick spacing

· F3: Cleft of heart spitting

Our Recommendations

In our opinion, it is very important for a diamond to have an excellent symmetry. This affects the general outlook of the diamond and ensures that the scintillation (flashes of light) that dances as you move the diamond are even and identical.


Polish examines the surface condition of the diamond, known as blemishes. Like symmetry and cut (proportions), it is graded from: Excellent, Very Good, Good, Fair to Poor.

Common types of blemishes;

  • Polishing Lines [1], Burn Marks [2], Lizard Skin

  • Scratch [3], Pit

  • Abrasion [4], Nick, Rough Girdle

Polishing lines are common in diamonds but most often they can not be seen with the naked eye and does not greatly affect the overall beauty of the diamond.

Burn marks, scratches and abrasions on the other hand should be avoided as it would make your diamond appear dull.

Our Recommendations

In our opinion, a diamond with very good polish on the condition that it is only due to polishing lines, looks no different than an excellent polish diamond. Hence buying diamonds with a very good polish is a nice cost saving option while keeping cut (proportions) and symmetry excellent. This way the premium price tag of the triple excellent diamond can be avoided, while having no obvious differences.

Also, one thing to note is when you purchase a excellent polish diamond, and say you set it into a setting and wear it often, it could eventually become a very good polish diamond, due to the possibility of scratching the surface during normal daily wear.


The amount of light returned is checked using an ASET Scope or Ideal Scope. This is not very often used in retail stores but some jewellers may do so to differentiate their diamonds. Do note that GIA does not measure light return, but AGS does so and may sometimes include an ASET chart for it as seen in the red circle on their certificate.

ASET Scope Readings

Red: Brightness, Blue: Contrast

Green: Less Bright, Grey: Light Leakage

Diamonds with less green and grey areas are a better choice as there is lesser light return.

Our Recommendations

In cost savings, we would recommend a diamond with very good polish instead of excellent as there is not much eye visible difference in these two grades. However, it is important that symmetry and cut are as kept as excellent. In this case, the diamond would still look as beautiful but you would have avoided the triple excellent premium.

Head back to our Ultimate Diamond Buying Guide to learn about the other components of diamond grading.


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