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                The origin of the colour in diamonds

Our brain and our eyes are the sensory equipment which enables us to see the colours. The colour can only exist that when there is an interaction between the light, the object in question and the human.

The white light i.e. the light which composes our entourage (visible light) are a combination all wavelengths of the electromagnetic spectrum. Each colour corresponds to a certain wavelength, thus giving all the colours of the rainbow.

In our most common language we do not carry distinction between the colour and the hue, however there is a distinction between the two. The physicists discovered for a certain time, the existence of 3 principal colours, each principal colour has a secondary colour, such as for example: yellow for blue, the cyan for the red and the magenta for the green. This fact does not import which colour can be made up of a combination of these six secondary colours. The illustration of this ellipse at circular base (A.Munsell), us describes the tone thanks to the vertical axis, the ray of the ellipse describes saturation and on the circumference one finds the colours. 

 

The purple , the brown, the white, but also the gray are neutral colours which are deprived of colours. The colours known as basic is the red, the orange, the yellow, the green, blue, the purple and the violet. When we described the colour of a diamond or in a gemstone, we describe a feeling, resulting from a complexe physiological process, here undoubtedly one of the reasons which would explain why it is so difficult to describe a colour and especially why this perception differs as much between every person.


The origin of these colours in diamonds can be explained by its atomic structure. The crystal lattice of a diamond is only composed solely of carbon atoms (C), placed in a three-dimensional structure. Each carbon atom is related to 4 other carbon atoms, thus forming a tetrahedral arrangement. A perfect diamond, only made up of carbon, will be colourless or near-colourless.

 

 

In a general way, the selective absorption certain wavelengths of the electromagnetic spectrum of the white light, and thus the transmission of remaining cause the colours in the objects or in the gems. These absorptions correspond to the colours which one observes, they can be had with the presence of impurities (nitrogen, boron, hydrogen, nickel) for diamond, or of defects (coloured center). The presence of boron can make a natural blue diamond, the presence and nitrogen give the yellow. In fact the pink, the red, the purple one and the grey can be caused by the presence of hydrogen and finally the green one can come from the contact of diamonds with certain radioactive uranium salts (natural radiation). These natural green diamonds are without danger for the human and they are not radioactive. 

The presence of inclusions in diamond can also make the colours,  there will be then diffusion by impurities or being related to physical causes such as the plastic deformation, or more rarely on coloured centers: defects associated with hydrogen.

In fact the cause of the brown colour, but also of the pink colour, can be the result tensions in the crystal identified under the name “graining” coloured (brown or pink) or colourless. This term is used in gemmology, which refers to parallel plates between them in a colourless or coloured matrix.

Diamonds have spectral properties especially when we observes their behaviour under a lamp with ultraviolet ray and in the field of the infra-red, this allowed the physicists and researchers to divide them into two categories, diamonds of the Type I and Type II. Certain coloured diamonds can be from the Type I (with nitrogen) or with Type II (without nitrogen).

Below we propose a summary by colour  of the causes of the colour in a naturally coloured diamond:

Natural color                                    Causes of the color

RED

PINK

BROWN

Center unknown related to the deformation
plastic “graining”


Photograph: pictures of the graining in a natural pink diamond
Courtesy: Benjamin Rondeau

Nitrogen insulated trapped by a gap
Varied defects related to hydrogen

PURPLE

Defects related to hydrogen
BLUE

Boron traces: atom within the structure
Defects related to hydrogen
Natural irradiation: carbon gaps
GREEN

Natural irradiation: carbon gap
Fluorescence: green luminescence of the H3 center
Defects related to hydrogen
Coloured centers : Not identified
YELLOW

There are 6 causes of the colour:
Aggregate of 3 nitrogen atoms: Center N3
Defects implying hydrogen or nitrogen
Nitrogen insulated within the network
ORANGE

N3 + gap trapped by a nitrogen aggregate
Nitrogen insulated in network
BLACK

Black or gray inclusions: carbon graphite, etc..
Great number of fractures or diffusion centers
GREY

Defects related to hydrogen
Boron traces

WHITE

Micro unknown inclusions



 

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Did you know ?
The De Beers detain a spectacular collection of natural coloured diamonds, all colours of the rainbow compose it.


 
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