Cut of Diamond


Cut of diamond is the most critical of the 4C’s, the variables governing value of diamond and refers to the proportions, symmetry and polish of diamond. Diamonds are a transparent gemstone and dependent on the size of facets to diamond, interacts with transmitted light to create internally reflective, dispersive, refractive and scintillation brilliance.

The term cut must not be confused with the actual shape of diamond which refers to the outline of varying shapes of diamond such as Round Brilliant, Cushion, Emerald, Asscher and Princess cut diamond.

The term cut of diamond refers to the only variable of the 4C’s which is influenced by hand, as Carat weight, Colour and Clarity are all governed by nature via the billion year growth process of rough diamond. To understand cut in more detail, the polisher has a crucial role in realising the ideal diamond cut which is decided by the original shape of the rough diamond, location of internal flaws or inclusions, the preservation of carat weight and popularity of certain shapes amongst consumers. The polisher must consider each of these variables before proceeding.

Rough diamond and yield of polished diamond


The polisher has to decide whether to yield from the rough crystal above, the ideal proportions for a diamond to realise maximum brilliance , as per Ideal cut above or compromise with a spreadier, heavier weighted Standard cut with a smaller crown and weaker brilliance.

So diamonds are actually cut to preserve as much weight as possible as a heavier carat weight could be more attractive to a consumer however the cost per stone increases and the cut grade would be weaker due to deviation from ideal proportions. If the polisher adhered to ideal proportions a better cut grade would be issued by a grading laboratory. Cut grade is so important as it allows the buyer to identify diamonds that have not been cut to ideal proportions.


Diamond proportions

A diamond cut has to have harmony between diameter, crown height, depth, crown, pavilion angles and girdle, referred to as proportions. The table and depth percentages both represent a percentage of the girdle diameter and also affects how light travels within diamond which can impact on the brilliance of diamond.

If you revert to ideal cut above the ideal proportions allow light to strike each pavilion facet at an angle to reflect back to crown and light refracts on exit enabling dispersive brilliance.

If you revert to cut too deep light strikes the first pavilion facet at an angle sharp enough to reflect to the second pavilion. However, the light strikes the second pavilion at too low an angle, causing the light to refract through the bottom of the diamond, a diamond will appear dull or dark.

If you revert to cut too shallow, light strikes the first pavilion facet at a low angle and passes through facet and refracts through bottom of diamond, a shallow cut diamond will lose brilliance.

A well proportioned diamond will have each facet properly placed and angled so as to maximize the amount of light that reflects back out of the crown of the diamond to viewer. This reflected light is called reflected internal, dispersive (fire) and scintillation brilliance (sparkle).


GIA Grading Reports

To understand diamond cut we all know that diamonds are renowned for their ability to transmit light and sparkle. GIA laboratory services take a view a diamond’s cut grade is really about how well a diamond’s facets interact with light.

To determine the cut grade of the standard round brilliant diamond, the shape that dominates the majority of diamond jewellery, GIA calculates the proportions of those facets that influence the diamond’s face-up appearance. These proportions allow GIA to evaluate how successfully a diamond interacts with light to create desirable visual effects such as:

Brightness: Internal and external white light reflected from a diamond.

Fire: The scattering of white light into all the colors of the rainbow.

Scintillation: The amount of sparkle a diamond produces, and the pattern of light and dark areas caused by reflections within the diamond.

GIA’s diamond cut grade also takes into account the design and craftsmanship of the diamond, including its weight relative to its diameter, its girdle thickness which affects its durability, the symmetry of its facet arrangement, and the quality of polish on those facets. These two variables are called the finish of diamond which I will address in another blog.

GIA employs cut grades ranging from Excellent/Very Good/Good/Fair/Poor. Excellent Cut is granted to diamonds with the highest level of brightness, fire and scintillation while the lower gradings of cut are granted to stones with poorer light reflection, fire and scintillation.

GIA only grade cut for round brilliant cut diamonds and not fancy cut diamonds due to the nuances in proportions and measurements which can vary considerably.

At Graham Tom, we only sell diamonds for our bespoke jewellery services  accompanied with either a GIA dossier or a GIA grading report as we believe in the consistency and accuracy of GIA’s grading standards.


Is your diamond on a GIA report a borderline stone?

Several clients have contacted me recently to raise a valuation for insurance purposes. They come into the Bourse waving their GIA reports with a satisfied smile. They explained that their bridal focal diamond was bought online at a great discount off Rapaport list or from a boutique in a mall in the Emirates at a great price. It is at this point alarm bells start to ring before I even start to gauge diamond measurements, proportions and assess mount. Why you may ask?

The diamond market currently is in a quandary where the price of rough diamonds offered at tenders/auctions are too high for diamond manufacturers to yield a profit and “sights” are being deferred to later dates. Polished diamond manufacturers are loading their inventory with very tight margins onto Rapnet, the largest diamond trading platform targeting wholesalers, diamond brokers and retailers.

Borderline report for Cut 1.00 ct
Borderline report for Cut 1.00 ct with weak measurements for stone
Better measurements for Cut 1.00ct
Better measurements for Cut 1.00ct stone

You then have an open market of willing buyers for bridal and lifestyle purchases of diamond jewellery with an eclectic mix of sellers of diamond jewellery all vying to be of service with access to Rapnet , including e-tailers, bricks and mortar retailers and boutique operators. But the market can arguably be closed for the diamond manufacturers especially in India and China as there is no liquidity to purchase goods and margins are being squeezed. As a result these diamond manufacturers try and maximise yield from the rough diamonds which are then sent off for grading to GIA laboratories. But when it comes to “make” of diamond which manifests as Cut, Polish and Symmetry on GIA reports, these diamonds are actually borderline stones.

Both diamonds identical weight with different Spread
Both diamonds identical weight with different Spread
Left stone has very thick girdle
Left stone has very thick girdle and is deeper


I say borderline because the ideal measurements and proportions of an ideal cut can be compromised. For example above, if you take two 1.00ct F/VS1 carat brilliant cuts,(see reports 1a and 2a top) the diameter on an ideal cut should range from 6.35mm to 6.50mm (see larger spread right stone 1b) but a borderline diamond on a GIA report can easily have a diameter of 6.15mm-6.25mm and will appear considerably smaller against an ideal cut (see left stone 1b) These borderline diamonds also have a very thick girdle (see first stone to left 2b ) which can affect brilliance of diamond. The proportions on reports below left also highlight how deep the stone is with measurements which have been compromised. Borderline diamonds trade at a very significant discount and are attractive to willing buyers on a tight budget.

Borderline proportions 1.00 ct
Borderline proportions 1.00 ct. Thick Girdle/Deep stone
Better proportions for 1.00ct stone
Better proportions for 1.00ct stone Thin/Medium girdle/Better spread

You also need to factor in clarity as quite often if a buyer requested an SI1 or SI2 diamond, a borderline diamond will have the inclusions to centre of table which can be seen by naked eye and not scattered to girdle or to pavilion. Again these stones trade at a very significant discount. Colour can also be borderline , remember diamond grading in all laboratories is subjective and not definitive and represent an opinion of the grader.

My point here is that in an ideal scenario, the willing buyer has choice where to purchase diamond jewellery. The buyer today is better educated with the 4C’s and information is freely conveyed online so please ensure you do your homework before you move forward. Make sure you buy from a reputable jeweller who has experience in the industry and do not get taken in with an offer of diamond at a cheap price online or in the malls as you will regret your borderline stone purchase on a GIA report should you wish sell it a later date.

If you require any help with any of our services or have any other concerns I will be delighted to assist you.
Please call me on 0207 404 4022 / 07785 730 214 or leave a message on the contact page

Do you have an EGL Certified Diamond from a reputable laboratory?

EGL Certified Diamond

Over the last two weeks, there has been much noise within the industry over the credibility of honest grading from one of the franchises of the diamond grading laboratory EGL (European Gemmological Laboratory ) which has impacted on retail, the end of the Jewellery supply chain. This relates to EGL certified diamonds.

The CEO of Rapnet, a universal trading platform for diamonds which provides a valuable source for diamond brokers, has taken a stance and has refused to play host to all EGL certificated diamonds uploaded by diamond cutters and wholesalers. The reasoning is that one of the franchises, EGLI (European Gemmological Laboratory International)  has been subject to criticism over the last few years for over zealous diamond grading.

EGLI have been accused of using the Gemmological Institute of America (GIA) terminology to describe diamonds as four or more colour/clarity combination grades higher than what the GIA would have given to the same diamonds. The impact of this practice has facilitated a profit motive for retailers to sell for example an EGL certificated E/VS1 diamond to consumers at a similar price or more economical price than a G/SI1 GIA certificated diamond.

The president of the World Federation of Diamond Bourses (WFDB) has argued that this unethical standard of trading by retailers and the acceptance and support of a culture that promotes misrepresentation of diamond quality and the outright cheating of consumers destroys the diamond trade from within. The elimination of this practice should be addressed and any members of Bourses that participate in this fraud will be answerable and may be expelled.

As a member of the WFDB, I have always promoted the exacting standards of diamond certification by GIA and believe their grading standards has a set a benchmark for other laboratories to follow suit, although some may have differing nomenclature. The GIA created its gemmological standards and related terminology in 1953 and issued diamond grading reports in 1955. GIA has continuously and consistently supported its standards by issuing grading reports for millions of diamonds.

EGL Certified Diamond

Summary of diamond grading laboratories

I would comment  as a diamond broker based in the UK, 90% of all diamonds bought by my clients are supported by a GIA certificate but occasionally,  a particular specification of diamond is requested by a client at a given time and is not available on GIA.  I will often recommend other laboratories such as HRD (Hoge Raad voor Diamant” translated as “Diamond High Council”)

in Antwerp and IGI (International Gemmological Institute.) The latter is franchised around the world and in the USA,  the AGS (American Gemmological Society)  offers exacting standards for diamond grading reports and would not hesitate in recommending  all these laboratories .

So anyone embarking on a journey to find the ideal diamond for yourself or your partner please be aware of retailers selling diamonds at very attractive prices on lesser known laboratory certificates. Make sure your EGL  certified diamond is a credible one. Do spend the time, exercise due diligence and really consider the benefits of buying a diamond on a credible laboratory certificate. You will find the value will give you peace of mind and outweigh the difficulty of trying to resell or upgrade your lesser known laboratory certificated diamond at a later date.

The 4C’s: Variables governing value of diamond

The 4C’s are a common standard in the diamond and jewellery industry representing Cut, Colour, Clarity and Carat weight. They were introduced by De Beers over 50 years ago to help consumers understand the value of a diamond.


Cut is a term used twofold to either describe the shape of a diamond or the ideal proportions of a diamond. It is the only C influenced by hand as the other variables are governed by nature.


There are varying shapes of cut of diamond including round brilliant, princess, asscher, emerald, heart, pear, oval, marquise, radiant and cushion.

A diamond polisher’s challenge is to deliver from a rough diamond a polished diamond that offers maximum brilliance and fire reflected back to the viewer. To achieve this, a polisher has to adhere strictly to the ideal mathematical proportions of each shape which are applied to the three main components of a diamond, the crown, girdle and pavilion.

If a polisher adheres to the ideal proportions of a diamond shape an ideal cut grade will be issued and if there is deviation away from ideal proportions, a weaker cut grade will be issued from diamond grading laboratories.


Colour is a term used to describe the subtle difference between the colourless grade of D-F to the near colourless and off white diamonds which then appear to draw a very slight tinge of yellow which increases to a more noticeable yellow at the end of colour scale. Grading of colour is performed under controlled lighting usually with a master diamond colour set for comparison to help accuracy in determining colour.



When the intensity of colour in a diamond is stronger than in the scale above, a diamond may be given a ‘fancy colour’ grading. Fancy colour diamonds occur in shades of brown, black, yellow, orange, green, pink, blue and red. Fancy colour diamonds are increasingly sought after.


Clarity is a term used to describe the presence of natural inclusions visible within a diamond which occured in the crystallisation growth of diamond. For purposes of grading clarity, a 10 x diamond loupe (magnifying glass) is used to determine clarity grade which can vary from Flawless to Included.



Inclusions in a diamond may include cracks, fractures, carbon, clouds and natural growth lines. Diamond grading laboratories indicate the location of inclusions in a diamond which can vary from centre of the stone to the girdle.

Carat Weight



Carat is the measure of weight for diamonds and needs to be distinguished from the size of diamond, which is affected by cut and shape of the stone. There are 100 points to 1.00 carat which is equal to 1/5 of a gramme. If you hold colour and clarity constant and increase the carat weight of the diamond as per the illustration above, the price of the diamond increases dramatically.

The 5th C is often used to refer to the Jeweller! Another subject in itself!