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In an article published this month in the international journal Diamond and Related Materials*, research workers from the United Kingdom and Japan describe a series of measurements they have carried out to understand the change in colour of natural diamonds, from a cheap and nasty brown to an exciting fancy yellow/green, produced by the NovaDiamond high-pressure, high-temperature (HPHT) process.
Details of this work were revealed in a lecture "The colour of diamond and how it can be changed" presented by Professor Alan Collins at a packed meeting of the Gemmological Society of Great Britain in London on 19 April 2000. Collins also explained how suitably chosen colourless diamonds can be converted into attractive "canary yellow" stones by the HPHT process.
The research involves optical absorption and luminescence spectroscopy, and the authors claim that the spectra of the diamonds are unlike those observed for natural diamonds. Consequently it should be straightforward to differentiate a natural yellow/green diamond, or a natural canary yellow diamond, from those produced by HPHT processing.
The work has been carried out by Alan Collins, Professor of Physics at King's College London, Dr Hisao Kanda of the National Institute for Research in Inorganic Materials, Tsukuba, Japan, and Hiroshi Kitawaki of the Gemmological Association of all Japan. Professor Collins is a leading international authority on colour centres in diamond and Dr Kanda is a high-pressure expert who has spent many years producing diamond by the HPHT process and carrying out HPHT processing of diamond.
The authors argue that the brown colour of natural diamonds is associated with plastic deformation. The plastic deformation was probably produced when the diamonds were brought to the earth's surface by volcanic eruption, and results in a distortion of the regular array of atoms that make up the crystal. The diamonds show clear evidence of internal strain, and exhibit "coloured graining" in regions where the planes of atoms have slipped past each other, producing dislocations.
When the diamonds are subjected to temperatures of approximately 2000 degrees Celsius (3600 degrees Fahrenheit), with a stabilising pressure of approximately 60000 atmospheres, some healing of the dislocations, and possible re-growth of diamond, occurs. This leads to a reduction in the brown colour.
If the diamonds have a very low concentration of impurities (like the so-called "type IIa" diamonds used by the General Electric / Lazare Kaplan consortium to produce GE-POL stones) the end result is simply an improvement in the colour grade of the diamond.
The specimens processed by NovaDiamond are mainly the so-called "type Ia" diamonds which contain typically a few hundred parts per million of nitrogen. In most natural diamonds the nitrogen is present as pairs, or a group of three nitrogen atoms surrounding a "vacancy" or a group of four nitrogen atoms surrounding a vacancy. (A vacancy is a missing atom from the diamond crystal structure.)
Heating brown type Ia diamonds under the conditions described above produces two effects:
1. When the dislocations begin to heal, additional vacancies (V) are released and are trapped by the nitrogen pairs (N-N) to form N-V-N centres.
2. Some of the nitrogen pairs dissociate to produce single nitrogen atoms. This allows a few N-V centres to form, and also affects the electrical "charge state" of the N-V-N centres.
Uncharged N-V-N centres produce an absorption in the blue part of the visible spectrum, and this gives the diamonds a fancy yellow colour.
Negatively charged N-V-N centres produce absorption at the red end of the visible spectrum; when this is combined with the absorption at the blue end of the spectrum the resulting colour of the diamond is fancy green or yellow/green.
Provided the nitrogen concentration is not too high, the absorption of light by the uncharged N-V-N centres produces green luminescence which is clearly visible in daylight or under illumination with a tungsten lamp, and this gives an extra vibrancy to the yellow/green colour.
If the starting material is colourless type Ia diamond containing nitrogen pairs (but no larger nitrogen aggregates), the dissociation of the nitrogen pairs produces some single nitrogen atoms which produce the absorption characteristic of "canary yellow" diamonds.
The authors have drawn on a huge range of previous research work on diamond, and this, together with their present measurements, has lead to a consistent explanation for the colour change process currently being exploited by NovaDiamond and others.