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Fancy Coloured Diamonds

 

Today, our beloved diamonds can be worn in a many different hues, and many of them are much more valuable than colourless or white diamonds. With fashions changing every season, and celebrities showing the way on the catwalk, we have now come to consider that pink, blue, green, yellow, brown, and even the dramatic black diamond are ‘de rigueur’.

The HRD Antwerp Diamond Colour Certificate

 

Every genuine diamond sold is issued with a certificate to prove its authenticity and provenance. In 1983, because of the increasing interest in fancy coloured diamonds, the HRD Antwerp Certificates Department responded by introducing the HRD Antwerp Diamond Colour Certificate, developed to take into consideration the exceptional colours and unique qualities of coloured diamonds.

As we have said, it is expected that every (natural) coloured diamond will have the accompanying certificate. The certificate will state where the diamonds originated and other attributes. Because of developments in technology, there are myriad ways of altering the colour of a stone. For some jewellery, this is an acceptable way to enhance the look of a piece, but a colour-treated diamond will not receive a Colour Certificate as this will state only that the colour of a diamond is its natural state. The High Pressure High Temperature process is acceptable.

A usual diamond certificate will place much emphasis on the 4Cs, including the clarity grade. A Diamond Colour Certificate is much the same, yet no clarity grade is included. The colour grade bears most of the emphasis.

A coloured stone can contain inclusions that are visible to the naked eye (clarity Grade P1 and I1), yet still be valued at hundreds of thousands of dollars per carat. This is the effect of the rarity value. Colourless diamonds with similar visible inclusions would not reach those values. Carat weight is also very important when valuing a coloured stone, but a natural pink, purple, red or green diamond will have a large price tag, simply because of its rarity and uniqueness. Clarity, Cut, and Carat weight do have a bearing on a coloured stone’s value, but it is the Colour Grading which has most impact.

Source of colour in diamonds

Most polished diamonds are colourless to yellow, with middle tones of light yellow and greenish-yellow. The stones are part of the Cape-series, the first source of diamonds found in South Africa. The fancy coloured diamonds, pink, red, green, blue also exist but are few and far between and are described as very rare.

It is the atomic structure that forms the many beautiful hues of a fancy coloured diamond. The colours originate in the atomic structure of the diamond. The structure that makes the diamond such a hard substance is the crystal lattice deep within the diamond made up of carbon (C), arranged three-dimensionally. Around each atom are four other carbon atoms which form a tetrahedron. An ideal diamond, built only of carbon would be completely colourless.

The reality is that the structure of diamonds will contain structural deviations and atomic impurities. Deviations from the ideal structure can be optically active and known as colour centre. So therefore it is these colour centres present within the stone that gives it its colour. One of the main defects will be a foreign atom that has replaced a carbon atom within the structure. Some colour centres do not contain foreign atoms but, perhaps, vacancies. Nitrogen can be found within a diamond and is the most frequently occurring impurity atom. Diamonds are divided into two groups depending on nitrogen’s incidence in a diamond lattice: type I that contain a quantity of nitrogen that is detectable, and type II for diamonds with virtually no nitrogen.

Type I diamonds ~ This group is the largest group and forms the majority of polished diamond, probably 95%. Nitrogen atoms (N) substituted for carbon atoms (C) is the main characteristic colour centre of this type. The N-atoms can be isolated or grouped, perhaps in twos, threes or sometimes more. Some nitrogen groups absorb blue light, resulting in the host diamond having a light yellow colour, modifying to brownish-yellow and brown. These groups of nitrogen atoms can be subdivided into additional groups.

  • Type Ia: In the crystal lattice of this group, the nitrogen atoms are organised in groups. The category is in turn separated in two: diamonds with groups of 2N-atoms (A-aggregates) and called type IaA; diamonds with groups of 4 N-atoms around a vacancy (B aggregates) are called type IaB. In most diamonds both A- and B- aggregates are present: this combination is usually referred to as type IaAB. Although these A- and B- aggregates are optically active in the infrared part of the spectrum, these stones usually also contain the N3-centre (3 nitrogen atoms encircle a vacancy) that causes the typical (Cape) yellow colour.
  • Type Ib: In this case, the N-atoms are not positioned in groups but individually spread and secluded in the crystal lattice. Diamonds with a proper intensity of secluded N-atoms show a distinct incorporation of light with wavelengths of less than 560 nm. I.e. they absorb blue and green light from the visible spectrum and so are observed as yellow-to-orangey-brown. Natural diamonds of type Ib are usually a deep yellow colour, known as ‘canary’. However, they can be brown or greenish depending on the intensity of nitrogen atoms and the effect of other colour centres. Type Ib natural diamonds are very rare, that is less than 0.1% of the total number of gemstone-quality natural diamonds. It is assumed that immediately after they form, all nitrogen containing type I-diamonds belong to this type. Nevertheless, after they form, most diamonds remain millions of years at high pressure and temperature conditions in the upper mantle. Due to these conditions, nitrogen atoms gradually group into aggregates and type Ib diamonds become type Ia. Synthetic diamonds are, however, always very young diamonds, which explains why they mainly contain a very high level of isolated nitrogen. However it is possible with a HPHT treatment to transform the stone from type Ib to type Ia.

Type II Diamonds ~ With nitrogen being the most prominent impurity, a type II diamond could be considered the most atomically pure of the mineral. These diamonds do not display characteristic absorption properties we would expect with nitrogen atoms.

Type IIa ~ A diamond that has no nitrogen within its composition and no impurities that could influence the diamond’s colour. These diamonds are often colourless, because there is also a lack of absorption in the visible spectrum. Alternatively, they may have a pink, red, or yellow-brown colour because of the presence of structural colour centres instigated by plastic deformation. Type IIa diamonds have a great rarity (only 1 – 2%). The famous Koh-I-Nor and the Cullinan are Type IIa diamonds.

Type IIb ~ In the crystal lattice of Type IIb diamonds, there are substitutions of boron (B), which is usually of a low content. The atoms of boron absorb light from the yellow, orange and red part of the visible spectrum, as well as the infrared. In reality, the diamonds display a blue colour, yet they can also be almost colourless or grey. The Blue Hope diamond is a prime example that belongs to Type IIb.

A ‘normal’ diamond is electrically insulating at room temperature. A Type IIb diamond is electrically conductive. Blue diamonds that are not conductive do not belong to type II and may well be synthetically coloured. A very small 0.1% of diamonds belongs to this group.
Table I shows the types of diamond and those diamonds that fall within that group:~

Colour Grading

Two main groups can be distinguished according to colour saturation and colour type of a diamond

  1. 1. Cape-colours plus equivalent colours
  2. 2. Less common colours – ‘Fancy’ colours

Colour grading of Cape coloured diamonds

All gem-quality diamonds (90 – 95%) are part of the Cape-coloured diamond group. The colours are in the range of colourless to light yellow. The colour grading of these stones are determined by comparison with a master stone set, produced by representatives of the World Federation of Diamond Bourses(WFDB) and the Confederation Internationale de la Bijouterie, Orfevrerie des Diamantes, Perles et Pierres (CIBJO). This group of diamonds is named ‘the mother set’, stored at HRD Certificates Department. Any subsequent sets must be in direct comparison to this original set.

The table below represents the International Colour Grading Scale, and is compared to the GIA system and some less recent colour nomination.

Colour grading of equivalent colours

This collection of diamonds shows a colour from the Cape group mixed with another very weak tone of colour of either green, grey or brown. They are graded in the same way (comparison to the same stones of reference) and are very close in colour to the Cape series. This method of grading of equivalent stones will depend on the depth of colour, and applicable to ‘tinted white’, L colour I.e. diamonds with low colour diffusion.

The name of this diamond will therefore consist of a Cape-colour with the addition of the denomination ECG (equivalent colour grading)

E.g. Exceptional white (D) ECG

If there is a more intense colour impression than ‘tinted white’ (L) it is graded as a fancy coloured diamond and not considered as slightly deviating.

Colour grading of fancy colours

Any diamond which consists of a colour other than the Cape-colours i.e. purple, blue, pink will be determined as a fancy coloured diamond. The word ‘fancy’ is used for a diamond which is out of the ordinary, extremely beautiful and memorable. There are many different shades of fancy diamond: They are not just within the group of primary colours, say red, green, blue, yellow etc, but in various hues.

A diamond’s colour description is made up of three fundamentals:~

  • Saturation – a term that expresses the intensity of a diamond’s colour and will define the depth of saturation, either weak or strong
  • Hue - a term which distinguishes between the colours of the rainbow, and those colours in between that dissolve into each other. White, grey and black are known as the neutral colours. Chromatic colours are those which do not have a hue.
  • Value – a term distinguishing light from dark and is used for both chromatic and neutral colours. The grade is a 100 value for white down to 0 for black.

In theory, possibilities of these parameters are endless... and rather complex.

HRD Antwerp uses an international standard of colour names, known as the Munsell Book of Colour. The Munsell system is made up of 1500 standardised colour cards based on the definitions put forward by the National Bureau of Standards which aim to give an accurate description of colour, using the three main parameters. They determine the saturation, value and hue.

The Munsell colour cards are used to determine an accurate visual interpretation of fancy coloured diamonds. The diamonds is view from the crown (top of the diamond) with a colour grading perpendicular to the table. A card will be selected that fits the colour most accurately, and the colour name will then be selected. The Munsell Book does not supply colour names for Colour Certificates, but is the foundation for the colour used at HRD Antwerp.

The HRD Antwerp Colour Certificate (fancy names)

Hue ~ The colour of the stone as compared to the Munsell grading system. It may be one colour i.e. pink, or a combination of two, where the second colour name is the main colour. (greeny-blue or pinkish-red)

Intensity ~ The saturation or depth of the hue. HRD utilises various prefixes to illustrate the intensity.

  • Faint – e.g. faint-blue. The colour hue cannot be seen through the crown and is only faintly distinguishable through the pavilion.
  • Light ~ e.g. light-pink. The colour hue can be seen through the crown is clearly distinguishable through the pavilion.
  • No prefix ~ e.g. red. The colour hue can be clearly seen through the crown and through the pavilion.
  • Intense ~ e.g. intense green. The colour hue can be seen clearly through the crown and pavilion and is of average to high value
  • Dark ~ e.g. dark purplish-pink. The colour hue can be seen clearly through the crown and is of low value
  • Translucent ~ e.g. translucent white. A stone which transmits light but with reduced transparency. There is no specification of intensity.

The above prefixes and their accuracy are crucial to determine the value of a diamond. A stone less than three carats and ‘faint’ and ‘light’ are not exceptional and will not command a high price, sometimes trading in the same grouping as ‘nice white diamonds’. The mounting of a diamond such as this will be detrimental to the colour. ‘No prefix’ through to ‘intense’ grading will produce the highest value of fancy coloured diamonds. They are finely balanced, determined by the purity of their colour and the depth of saturation with no degradation brought about with the presence of brown or grey.

Although ‘fancy’ is a well known term, and is usually applied to diamonds with outstanding attractiveness and beauty, there are a number of conditions the stones must fulfil:~

  • The diamond must be a natural stone. A diamond that has been treated in any way will not be considered as ‘fancy’
  • The diamond’s colour must be considered ‘pure’, with little grey in the mix. Black, grey or milky white stones will never be considered ‘fancy’.
  • A prefix of ‘intense’, ‘no prefix’ or ‘light’ must be present in the fancy colour name. Equivalent coloured diamonds and cape-type diamonds can be called fancy if they have an intensity of ‘intense’ or ‘no prefix’, and little grey.

Diamonds that have a colour within the grey zone can have the nomination of ‘fancy’.

Chameleon diamonds

Chameleon diamonds are, as the name suggests, diamonds that can change colour under certain circumstances. The colour change can be instigated by keeping the diamond in the dark for a period of time, or by heating. Most diamonds that are termed chameleon are discovered by one of these two situations where a noticeable colour change is present...one that takes a few seconds only, and where the change has been so remarkable it will be commented upon.

The original pure colour is swiftly replaced by the mixed colour, a change triggered by ultraviolet components in the light. Gently heating the stone 350 degrees centigrade will bring back the original colour. Overheating will damage a diamond. Three well-defined chameleon diamond types have been identified:~

Yellow to olive green diamonds

Most chameleon diamonds discovered belong to this type, although it must be remembered that chameleon diamonds are rare. They are similar in features and are part of the Type IaA group of diamonds. Upon heating the colour changes from a mottled greenish-yellow that is dull to a pure fancy intense yellow. When cooling, the colour will revert to the previous greenish-yellow colour. Under a long wave UV (366 nm) they will present a bright yellow fluorescence and for some minutes, a phosphorescence.

The electronic state of the impurities is the instigator of the colour change, and research into the mechanics of the colour change is sparse.

Pink and brownish pink diamonds

Dr. J. Van Royen of the HRD Antwerp Certificates Department identified and described the second group of chameleon diamonds. Exposing Argyle pink diamond to intense ultraviolet light changes them to a brownish pink. After time, the original colour will be restored. Intensity and speed of the colour change will be different for each diamond. Some stones will change colour when exposed to normal daylight, caused by the crystal defects in the electronic structure that are in the original pink colour.

D/E to G/H/I diamonds

The third group was determined by observing a diamond with remarkable colour change to colourless from faint pink after being exposed to ultraviolet light. With gentle heating, the stone returned to its original faint pink colour. Some may consider this to be a colour change in the top range of the colour scale...a colour change from Rare White to White (F/G – H) to almost colourless. This can happen when stones are retrieved from an extended period in the dark. Some may consider that this is not a particularly special phenomenon, with the stone belonging to the ‘olive’ coloured chameleons in the near- colourless range.

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