Colour Grading Diamonds : How To?

Everything you need to know about Diamond Colour Grading

Why do we colour grade diamonds?

Colour in gemstones, and particularly in diamonds is a subject of great discussion amongst gem professionals. It is common for people outside of the diamond trade to misunderstand the relationship between diamonds and colour. Diamonds are usually perceived to be white or colourless. Truly colourless diamonds are the purest crystalline form of carbon, devoid of even the minutest impurities in its structure. The rarity of these diamonds makes them even more valuable than the average diamond found in Nature and are therefore unique treasures. In general, most diamonds used in jewellery contain trace impurities of other elements and are either borderline colourless or nearly colourless, exhibiting a hint of a yellow or brown tint.

A diamond’s colour is a very important indicator of its value and beauty. A pure white diamond equates rarity - and, as we are understand, rarity is an indicator of value. The beauty of a diamond is a quick measure of its fire or brilliance, which in turn is determined by how white or colourless it is.

Evaluating the subtleties of a diamond’s colour takes years to master and is usually performed by a trained gemmologist or an experienced jewellery professional relying on a standardised grading system. The consumer's role is to be cognisant of, and understand the nomenclature of diamond colour in order to appreciate the underlying value of a diamond. Read on.......

Colour in Diamond: Causes and Effects

The appearance of colour in a diamond is caused by the presence of trace elements such as nitrogen and boron at the atomic level in the crystal. Often too, natural radiation present deep in the ground can cause shifts at the molecular level. These mutations result in selective absorption of the colours present in visible light – and the corresponding transmission of the unabsorbed spectrum gives a diamond its signature colour

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Photo courtesy: De Beers Diamond Institute

The mutations present in a diamond's crystal structure result in selective absorption of the colours present in visible light – and the corresponding transmission of the unabsorbed spectrum gives a diamond its signature colour.

Thus, a pure white (colourless) diamond indicates the complete absence of trace elements and very little or no retention of the colour spectrum of white light passing through it. The above diagram explains this phenomena in a lucid manner.

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Fancy Colour Rough Diamonds. Courtesy: De Beers Diamond Institute

Natural diamonds are found in a myriad of colours (all colours of the rainbow in fact) besides the usual white, yellow or brown – and these unusual colours are commonly known as ‘fancy colours.’ Some of the rarer fancy colours being: red, pink purple, blue, orange, and green. 

With fancy-coloured diamonds, more colour saturation usually translates to higher prices. And the purest single colours can fetch an even higher premium.  

Grading of fancy-coloured diamonds is done with the help of specialised nomenclature and equipment, laying great emphasis on hue, saturation and colour tone.
 
The topic of fancy colours is vast and too complex to be covered in this article. This article covers only the standard colour range of D-Z, while a later article will explore the infinite charm of fancy-coloured beauties.

Colour Grading Diamonds

Colour grading is the process of evaluating the colour of a diamond and assigning it a grade based on a standardized colour scale.

The Gemological Institute of America (GIA) developed the most widely recognized colour grading system in the 1950s, using a letter scale too indicate the colour grades of a diamond. It is now widely used by jewellery professionals and diamond dealers worldwide

The GIA colour scale starts at D (colourless) and extends down to Z (visible yellow or brown).

D, E, and F colour diamonds are considered colourless with D colour showing no trace of colour at all while the E and F colours have the smallest traces of colour visible under magnification.

G and H colour diamonds are considered near colourless and have some tint of colour which is visible only when the diamond is placed on its side on a white card or surface. I and J colours have a slight face-up tint that’s visible when compared to higher colours placed beside them under white light.

 

Diamond Colour Grading under White Light

Diamonds as viewed against a colour scale showing a distinctly increasing yellow tint as the letters approach Z

Diamonds that are graded K, L, and M have a faint yellow or brown colour which is visible face up to the naked eye and vary only in the degree of intensity of colour. These colours usually set up looking white in yellow gold jewellery settings.

N through R colour grades have more noticeable yellow or brown colours that are visible even from a distance. Diamonds over five carats in these lower colours are popular due to their obvious price appeal.

How is Diamond Colour Evaluated?

Colour evaluation of diamonds is a skilled process requiring an experienced eye. To be accurate, it must be performed under controlled lighting conditions such as daylight-balanced illumination, using a master set of previously colour graded diamonds for comparison. Another common method is to do this in natural daylight, in the early morning or late in the afternoon when the brightness is even and consistent and mimics the ideal lighting condition for colour grading.

At first the diamond is placed on its side on a white card or an angular white background and compared with master stones. The final evaluation is made with the diamond viewed face-up, which is the position that it would be viewed when set in a piece of jewellery.

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Grading Diamond Colour with a set of master stones in controlled lighting conditions. Photo courtesy IGI Gem Laboratories

A complete colour evaluation must also consider the colour tone, hue and saturation of the diamond. Hue refers to the basic colour of the diamond, such as yellow or brown, saturation refers to the intensity of the observed colour, and tone refers to the lightness or darkness of the colour.
The perceived colour of a diamond can be affected by a variety of factors such as the diamond's shape, size, and cut, and very importantly the colour of the surrounding  walls of the room. 

Diamond Colour Grading Nomenclature

Colour nomenclature is the system of naming and labelling diamond colours. The GIA took a fresh approach with the colour lettering scale, starting with the letter D (to avoid confusion with the loosely defined A, B, C, AA, AB,... colour grades widely used during that time). Thus D, the highest colour stands for a completely colourless diamond, and the scale goes down to the letter Z, which represents a diamond with a visible light yellow or brown colour. The letters in between, E, F, G, …… K, L, and M, represent the various grades of near-colourless and slight-colour diamonds.

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Commonly found shapes of Diamond Crystals. Notice the gradual visibility of colour in the descending scale order: D-E, G-H, J-K, M-N. Photo courtesy: DeBeers Diamond Institute

In total there are 23 colour grades subdivided into 5 brackets as follows: D-E-F: Colourless G-H-I-J: Near Colourless K-L-M: Faint N-R Very Light and S-Z Light (yellow or brown)
The colour grading system is standardised based on the perception of colour under white light or daylight. Colour usually manifests itself in a diamond as a shade of yellow or brown. It is for this reason that diamonds are graded for their lack of colour rather than their visible or apparent colour. Another noteworthy point - a diamond's colour intensity is influenced by its size - the bigger the diamond, the more obvious its colour. While the differences may be subtle, they can result in dramatic price variations.
Despite the GIA's colour grading scale being widely accepted and used, other grading labs may have slightly different colour grading methods and scales with small variations in the nomenclature. These are well received and popular in specific regions of the globe.

How Does Diamond Price Change with Colour?

The colour of a diamond has a significant impact on its price. As a general rule, the more colourless a diamond (an absence of colour), the more valuable it is.

As you move down the colour scale, from near colourless to light yellow or brown, the price of the diamond typically decreases. As the colour of the diamond moves lower, it is interesting to note that the price of the diamond doesn't decrease linearly.

The difference in price between a colourless F colour and a near-colourless H colour can be significant, but the difference in price between a near-colourless J colour and a faint yellow L colour can be narrower. Likewise, the price difference between faint yellow M colour and a light-yellow N or O colour can be even less significant.

Colourless or white diamonds, which are graded D through F on the GIA colour scale, are the most valuable and desirable type of diamonds because of their rarity, fire and brilliance - resulting from their ability to split white light into its seven component rainbow colours.

On the other hand, slightly colourless or tinted diamonds, such as yellow, brown and grey are generally less expensive because of their abundance and are also less desirable due to their lower levels of fire and scintillation.

Importance of Colour On the Price of Diamonds

A diamond's colour grade is one of the main factors affecting its value, but it is not the the  most important. Size or carat has the greatest influence on price. There are other factors such as cut or clarity which also impact the overall value of a diamond. 
 
Carat weight, which also refers to the physical size of the diamond, is a key factor in establishing the price of a diamond. The relationship between size and price is anything but linear. A larger diamond will always be much more valuable than a smaller diamond of similar quality and cut.
 
Colour in my opinion is the second most important factor after size (carat weight)  to influence influencing the price of a diamond. Cut and then clarity contribute to make up the remaining 2Cs to determine the overall value of a diamond.
  
Cut which refers to the arrangement and proportions of a diamond’s facets, can have a significant impact on the price of a diamond. A well proportioned diamond will have better light performance and will look more brilliant and sparkling than a poorly cut one.
 
Clarity which refers to the presence of internal and external blemishes in a diamond, is also a factor in determining the price of a diamond. A diamond with high clarity, (few or no visible inclusions) will be more valuable than a diamond of lower clarity.
 
All these moving parts taken together determine the price of a diamond.
A diamond that is well-cut, of a high clarity, with a colourless or near colourless grading will always be a better store of value than a diamond that is poorly cut, low in clarity, and has a less desirable colour

What Are the Diamond Colours Used in Jewellery?

The most desirable diamond colours for use in jewellery are the colourless to near colourless diamonds, which for the sake of simplicity we shall call white and are graded D to I on the colour scale.
The versatility and timeless nature of white diamonds makes them the best choice for jewellery. They are most often used in engagement rings and special occasion jewellery.
These white 'appearing' diamonds are versatile in terms of today’s fashion trends. They are perfect for all types of jewellery and are often used in engagement rings and other special occasion pieces. There is a steady year round demand for white diamonds for jewellery use due to the popularity of white gold and platinum jewellery.
In jewellery, the popularity of lower colour diamonds varies with the popularity of yellow gold jewellery, and depends on market trends, as well as personal preferences, economic and cultural influences etc. Some consumers may prefer light tinted diamonds such as yellow, brown or similar hues as a compromise for a larger sized diamond or a higher clarity.
Of the three top colourless grades, F colour diamonds are the most sought after for jewellery use being in that ‘sweet spot’ price bracket  - as they are considerably less expensive than D colour diamonds while still having a colourless face up look..
As a result, F colours are always popular with discerning buyers, as they provide a good compromise between 'borderline' colourlessness and an attractive price discount over D or even E grades.

Fluorescence and Diamond Colour

Fluorescence refers to the glow or light emission that a diamond can exhibit when exposed to ultraviolet (UV) radiation. It is caused by the presence of certain trace elements such as aluminum, boron or nitrogen present in the diamond's crystal structure.
 
Fluorescence in Diamonds

The 4 levels of fluorescence in a diamond. Notice the characteristic blue glow.

Approximately 30% of all diamonds found in Nature emit fluorescence when they are exposed to UV light. Although invisible to the human eye, UV radiation is ubiquitous. Sunlight contains it and fluorescent lights emit it too. Blue is the most common colour of fluorescence in gem-quality diamonds, but other less often seen colours include - white, yellow, and orange! This means that if you take your diamond to a night club with UV lighting, there is a good chance your medium-strong fluorescent diamond will glow on the dance floor, isn't that cool?
 See also: Our guide to Insights into the Diamond Industry
It has not been fully established how fluorescence affects a diamond’s colour, and experts are divided on this matter. In my experience, having handled thousands of fluorescent diamonds over the years,  faint  to medium does improve the face up perceived colour of the faint yellow (K,L,M) grades. Fluorescence and its influence on the price of a diamond is discussed in great detail in our blog: fluorescence in diamonds.......good, bad or ugly?
 
Here is a handy tip- a strong blue fluorescence can often make a light-yellow (N,O,P,Q,R) diamond look closer to its colourless cousins in sunlight. Blue and yellow are colour opposites and tend to cancel out each other, so blue fluorescence can completely mask the yellow colour. Choosing a good face-up strongly fluorescent diamond can sometimes yield you a bargain of a lifetime.

Common Myths about Diamond Colour

There are several common myths about diamond colour that perpetuate in the diamond industry:

  1. ''The whiter the diamond, the better'' - While colourless diamonds are often considered to be the most desirable, price conscious consumers prefer diamonds with a faint yellow or even tinted brown colour, as they can be considerably less expensive than colourless diamonds of similar size and quality.

  2. ''Diamonds that fluoresce are of lower quality" - While fluorescence can have both positive and negative impacts on a diamond's appearance, it does not affect the diamond's grading or durability. However, fluorescence in itself may affect the desirability and hence the price of a diamond.

  3. "A diamond's colour and value can be enhanced through treatments" - Some diamond treatments like irradiation, high pressure high temperature (HPHT), or film coating are used to artificially enhance a diamond's colour. It may at first appear positive for the diamond, but any treatment that artificially interferes with the diamond’s natural colour renders it ‘treated’ with an accompanying significant drop in value.
     
  4.  "The GIA colour scale is the only accurate way to grade colour" - While the GIA colour scale is widely used and well-respected, other grading labs have their own standards and are better accepted in some markets and for certain categories of goods. A point to note is that colour grading is a subjective process and a level of inconsistency is to be expected and accepted.

Can a Diamond's Colour Change Over Time?

A diamond's natural colour is very stable and can remain so if there are no external stimuli or influences to disturb this stability. In rare cases, prolonged heat or long multi-year exposure to strong sunlight or radiation can cause subtle changes or fading. It might take a very large number of years for the fading process to be noticeable, depending on the wavelength and intensity of the light and the length of exposure.

Heat can also cause a diamond to change colour. Heating diamonds above 1100°C can cause them to turn yellow or brown over time. This is particularly true if the diamond has been exposed to elevated temperatures repeatedly.

It is also important to note that while a diamond's colour can change due to external stimuli over time, diamonds are extremely durable and can last for centuries if left undisturbed. 

Colour Enhancement in Diamonds

Colour enhancement treatments of diamonds are increasingly common and not easily detected except by trained gemmologists. Some colour treatments, such as irradiation and high-pressure high- temperature annealing (HPHT) can result in permanent changes to the colour of a diamond, while others, such as film coating, can be temporary and can be removed by abrasive cleaning or solvents.
 
Irradiation, which is the process of exposing a diamond to a source of high energy radiation, can be used to change the colour of a diamond. The colour change is permanent and can be detected using a spectro-photometer which measures the absorption of the light spectrum by the diamond.
 
HPHT treatment can improve the colour of some type IIa brown diamond to colourless. This treatment can be detected by a well-equipped laboratory.
 
A coating enhances a diamond's colour by the application of a thin layer of coloured substance to its surface. The treatment can be temporary and is removed with solvent cleaning or an abrasive rub down.
 
Although detecting colour enhancements in diamonds is not always easy, disclosing any enhancement or interference to the natural colour of diamonds is important and an ethical requirement, since this can greatly affect the diamond's value. Most reputable diamond grading labs and organizations follow strict guidelines when it comes to identifying and disclosing any enhancement to a diamond's colour. Any treatment of colour enhancement of a diamond is required to be labelled as such on the grading report.
 
It is also important to note that, while colour enhancement treatments can improve the appearance or desirability of a diamond, they rarely improve the value of the diamond as the diamond loses its label of natural rarity.
It could well be worth less than the same diamond that has not been treated.
 
The safest way to ensure that a diamond is natural and untreated is to purchase it from a reputable certified jeweller or diamond dealer.

Some Takeaways:

  • Of the 4Cs, colour is the second most important characteristic because the human eye detects the size (and brilliance) first and colour second.
  • The 'lesser' the colour, the higher the grade. As a diamond’s colour grade improves, so does its price.
  • The distinctions between the colour grades are generally far too subtle for the untrained eye to detect. In the colourless grades, these differences cannot be easily detected except under magnification. 
  • Round diamonds hide colour better than fancy shapes. Fancy shapes such as ovals, pear and marquise show more colour due to their shallower cuts and weight distribution.
  • Thus, the face up perceived colour of a graded G colour round diamond is often comparable to that of a F colour graded oval or pear shape diamond.
  • To maximise your budget, choose a 'borderline colourless' (F colour) or a ‘near-colourless’ (colours G-H) diamond because the naked eye will still not see visible traces of colour for these grades.
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Suresh H.

Written by Suresh H.

Respected international diamantaire and industry expert with a deep knowledge of the industry, GIA trained diamond grader, researcher and writer about all things related to diamonds.

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