The Ultimate Diamond Cut and Cut Grades Guide

The PriceScope Diamond Cut and Cut Grades Guide

Diamond Cut - Ideal Cut Round Diamond
Posted by jcrow 2.13 J SI2 Ideal Cut Diamond


When diamonds are unearthed, they do not emerge as sparkling gemstones. The potential that a diamond has must be unleashed by expert diamond cutters; the results of this process will heavily impact the beauty and cost of a diamond. The most important of the 4C’s of diamond buying, diamond cut dictates the beauty, light return, brilliance, fire and scintillation qualities of a faceted diamond. Cut quality relates to a diamond’s facet proportions, angles, depth and symmetry. The cut is the most important value factor because the best cut quality makes the greatest impact on a diamond’s appearance compared to the other 4Cs: Carat Weight, Diamond Clarity, and Diamond Color.

Note – Though often used interchangeably, diamond cut and diamond shape are two different properties of a diamond. Shape refers to the physical outline of the diamond (for example, a heart shape), while the cut is related to the diamond’s facet proportions and angles.



When we think of a diamond, we think of sparkle, fire and brilliance – all of which are the result of a good cut. Choosing a diamond is a balancing act of beauty and value and ensuring a reasonable spend on the cut is the first step towards finding a beautiful diamond.

A well-cut diamond will successfully reflect white and colored light into the eye, resulting in sparkle; you may hear this referred to as light return or light performance. A badly cut diamond is lacklustre and dull, entirely lacking the diamond’s most coveted qualities. You could select a high clarity, high color diamond, but without a good cut it would remain a flat and disappointing stone. Diamond Cut trumps the other 4Cs by transforming a rough diamond crystal into a sparkling gem.



An expert gemmologist will analyse the diamond under strictly controlled viewing conditions. Industry specific optical measuring devices are used to extract data pertaining to proportions and measurements. The diamond’s light performance (white light, fire, scintillation) is observed, as well as the symmetry of the facets, their positioning and the quality of the polish. The diamond technology, combined with visual observations of polish and girdle thickness, is used to determine an overall cut grade.


The GIA will award one of the following cut grades to round brilliant diamonds: Excellent, Very Good, Good, Poor.

Diamond Cut - GIA Cut Scale : Excellent, Very Good, Good, Fair and Poor
GIA Cut Scale. Image Source: GIA


AGS uses a numerical system from 0 being the best cut to 10.


Diamond Cut - AGS cut grade scale uses numerical 0 being the best to 10
AGS cut grade scale. Image source: AGS


The GIA, AGSL, IGI, EGL, and HRD labs give definitive cut grades for round diamonds



There are many methods for assessing diamond cut, below are some of the well-known ones.

Helium Scanner/Sarine/OGI:Diamond Cut scanner

Proportions are measured with a Sarine, Ogi, or Helium scanner. Old systems use the worst proportion as the ‘grade maker’. AGS used this flawed method until May 2005.

Pros: Sarine reports are user-friendly and can give a good insight into the diamonds light performance.

Cons: These tools can narrow your search, but are not for final selection. Symmetry and other factors also need to be considered.


Ideal-Scope:Diamond cut - ideal scope

Ideal-scope uses a black lens and red reflecting material to indicate light return (red), contrast (black) and light leakage (white).

Pros: A convenient method for direct assessment, Ideal-Scope viewers and photos show leakage and light return. A simple tool for assessing optical symmetry and brightness.

Cons: Fairly basic and not good for fancy shapes.


Hearts and Arrows viewers:Diamond Cut - Hearts and Arrows Image

A black base and a conical indentation – the diamonds are viewed through the eye-piece. Both desktop and handheld versions of H&A viewers are available.  More on hearts and arrow…

Pros: Indicate the optical symmetry of a round brilliant, specifically the H&A patterning

Cons: Some H&A’s do not score <2 on HCA. Optical symmetry does not always equal the best light return.


ASSET:Diamond Cut - ASSET Image

An ASET scope works in a similar way to Ideal-scope. The colors contrast (blue), intense light return (red), indirect light return (green) and light leakage (white/black)

Pros: ASET gives a more detailed view of light return than Idealscope, highlighting the intensity of light being returned to the eye. Take a closer look here.

Cons: They are not as easy to understand. You can read more about the issues here.


DiamCalc:Diamond Cut - DiamCalc

Cut gemstone modelling. A photorealistic image of a polished diamond is produced to determine cut properties. These images can then be used with gem adviser software.

Pros: A virtual 3D model from a Sarin or Helium scanner can be viewed and tested with a free GemAdviser download. 3D methods give new R&D Ray Trace Cut info and virtual ideal-scope images.

Cons: There is a price to purchase software beyond the demo – for the average buyer, this spend may outweigh the benefits of the software itself. The full version of DiamCalc is targeted at diamond distributers/sellers.


AGS Laboratories has developed the first comprehensive light performance cut grading system utilizing sophisticated computer ray tracing.  It is used to grade round diamonds as well as a range of fancy shapes including princess, oval, emerald cut and certain cushion designs. For more information on the AGS Lab fancy shape diamond cut grades please visit: AGS Light Performance Cut Grade


Diamond cut proportions impact on how effectively the diamond reflects light. The image below outlines the anatomy of a diamond. You can click Anatomy of a Round Brilliant Diamond for more information about the basic facet structure.

Diamond Cut - Ideal Cut Round Diamond



A step when considering proportions will be to consider the depth of the diamond – cut too shallow the light hits the pavilion at a low angle and cannot be reflected through the table. Shallow diamonds may also display a ‘fish-eye’ effect. Cut too deep and the light hits the pavilion at an angle that allows it to leak through the bottom of the diamond-like a window. As a result, the stone will be dull, lifeless and is likely to appear smaller than an ideal-cut diamond.

The sizes, angles and percentages of each individual facet are taken together when assessing the proportions of a diamond. There is no ‘one size fits all’ approach to proportions, rather a range of combinations that can produce successful light return and beauty.

The Ideal-Scope reference chart below illustrates the effect of proportions on light return. Well-cut stones show maximum brilliance while shallow or deep stones have zones where light escapes and is not returned to the viewer’s eye. To learn more about using this diamond tool, please visit: Ideal-Scope Guide

Ideal-Scope reference chart for diamond cut proportions

Ideal-Scope Reference Chart ©2003 Ideal-Scope


The width of a diamond is measured from one side of the girdle to the other. Therefore the width of a round diamond is also its diameter.  Determining a length to width ratio is important when looking at fancy shapes – the length to width ratio indicates how wide or narrow it will appear. There is a movement for personal taste with the L:W ratio of some fancy shape – you may prefer a particularly elongated pear shape while others look for the recommended proportions (1.45-1.75). This ratio is calculated L÷W = L:W

Table %

The diamond table % is calculated by dividing the size of the table by the width of the diamond. The size of the table can determine the amount and quality of light that is returned to the eye.  Very large tables can produce excess glare which detracts from beauty.


The facets affect how well a diamond can refract and reflect light to create brilliance and sparkle. The symmetry, size and placement must be proportioned in order to create the desired optical effects.


The girdle may be small, but it is a significant part of the diamond that can affect cut quality and durability. The girdle is measured using highly sensitive scanners and is expressed as a percentage of the average diameter. An extremely thin girdle will be prone to chipping while a very thick girdle adds weight to the diamond with zero visual benefits. If a diamond has extreme variance in girdle thickness, it is usually an indicator of poor symmetry resulting in performance issues.


The quality of the polish is the difference between bright, mirror facets that reflect light, and dulled lifeless facets that inhibit reflection. These finishing touches are important to the overall quality and beauty of the diamond.


Diamond Cut Conclusion

In case we haven’t hammered it home quite enough, diamond cut is extremely important. A quality cut should be the first thing you look for when you begin your diamond search.

The following features are all impacted by diamond cut, and subsequently impact upon the price of a diamond:

  • Proportions (width, table, depth, angles)
  • Facets (symmetrical, proportioned and positioned to best reflect light)
  • Fire (colored light)
  • Brilliance (white light)
  • Scintillation (flashing/sparkling effect)


Cut is a complex aspect of diamond buying as there are so many factors at play. We advise using our HCA tool to reject poorly cut diamonds and narrow down your search. You can also use our fancy shapes guide to find the best proportions and cuts for fancy diamonds. Our network of experts can help you navigate the specifics of diamond cut down to the finer details.

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