Jewellers' Network

Fancy Coloured diamonds – by Kaylan Khourie, FGA

Coloured diamonds are rare gems found in every colour imaginable.

Each colour has a different cause and can be created naturally during growth underground or via laboratory treatments. Laboratory-grown diamonds can also be grown as specific colours or treated post-growth to create the desired colour.

Colour in diamond is caused by the selective absorption of light that is affected by things such as impurities, crystallographic defects and/or deformations – which can form naturally or can be created in a laboratory.   

The presence of specific impurities, defects and deformation features will result in the diamond absorbing light at specific wavelengths. This will leave the residual wavelengths of light to transmit through the stone and that is the colour that we see.

Yellow –

Nitrogen is the most common cause of colour in diamond and yellow is the most common colour – and is due largely to nitrogen impurities in the diamond’s crystal lattice structure.

Below, left to right

  1. HPHT laboratory-grown rough (as-grown) Diamond
  2. HPHT-treated Natural Diamond
  3. Natural orangy Yellow Diamond.

Pictured below are the three main nitrogen defects that can be present in diamond:

  1. Isolated nitrogen (known as C-centres); 
  2. nitrogen pairs (known as A-centres) and 
  3. four nitrogen atoms surrounding a vacancy (known as B-centers). 

The large majority of diamonds possess all three nitrogen defects in abundance.


Brown –

Brown is the second most common colour. The typical causes are due to nitrogen, vacancy clusters, plastic deformation, platelets and defects called ‘amber centres’; however the cause of brown colour in diamond is not yet fully understood.

Below: Brown graining caused by plastic deformation


Blue –

Blue diamonds owe their colour to either boron impurities in the diamond’s crystal lattice or irradiation.

Below:

  1. Artificially irradiated Blue Diamond coloured by GR1 centres,
  2. Natural Blue Diamond coloured by Boron impurities

(courtesy of Rez Diamonds).

The GR1 centre forms when carbon atoms are displaced by irradiation leaving a vacancy


Green 

Green diamonds owe their colour to nitrogen, irradiation, nickel impurities and/or green luminescence created by other defects related to nitrogen and vacancies (namely H2, H3 and H4 – pictured below).

Below:

  1. HPHT-treated Natural Diamond,
  2. HPHT laboratory-grown Diamond subjected to post-growth irradiation,
  3. Natural Green Diamond

(courtesy of Rez Diamonds).

Defects responsible for green luminescence


Orange –

The colour in Orange diamonds is not fully understood but many owe their colour to isolated nitrogen and plastic deformation.

Below:

  1. HPHT laboratory-grown (as-grown) Diamond,
  2. Treated (coated) Natural Diamond,
  3. Natural Orange Diamond

Photo credit: Taryn Willows


White-

White diamonds, which are not the same as colourless diamonds, are milky and owe their ‘colour’ to the reflection of an abundance of submicroscopic cavities filled with nitrogen precipitate.


Purple, Violet  and Grey  

Purple, Violet and Grey diamonds owe their colour to an abundance of hydrogen impurities.

Below:

  1. Perfect hydrogen cloud creating a Grey colour
  2. Violet and Purple Diamonds

Courtesy of Naturally Colored


Pink and Red  

Pink and Red diamonds owe their colour to plastic deformation and nitrogen-vacancy defects (pictured below).

Below:

  1. Natural Pink Diamond (courtesy of Rez Diamonds).
  2. HPHT laboratory-grown Red Diamond subjected to a multistep post-growth treatment exhibiting a typical cross-shaped fluorescence under short-wave UV light,
  3. Multistep Treated Natural Red Diamond.

Defects responsible for Pink/Red colour


Black 

Black diamonds are actually just diamonds full of dark inclusions (such as iron, magnetite, sulfides and most commonly – graphite) or are just very dark green, blue or brown.

Below:

  1. Natural – coloured by an abundance of natural radiation stains. Reflected light and under magnification.
  2. Treated – induced graphitisation process (a type of heat treatment). Reflected light and under magnification.


The author of this article, Kaylan Khourie, is a FGA qualified gemmologist and laboratory gemmologist at EGL South Africa.

All rights reserved. This article is the sole property of Kaylan Khourie and may not be reproduced by any form or means without the express written consent by Kaylan Khourie. Unless otherwise stated, all images have been captured by and belong to the author.

All defect diagrams are courtesy of Trevor Willows.