Brown diamonds

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A collection of brown diamonds in the National Museum of Natural History, Washington, D.C. The pear-shaped pendant at the bottom of the necklace weighs 67 carats (13.4 g). National Museum of Natural History Gold Colored Diamonds.JPG
A collection of brown diamonds in the National Museum of Natural History, Washington, D.C. The pear-shaped pendant at the bottom of the necklace weighs 67 carats (13.4 g).

Brown diamonds are the most common color variety of natural diamonds. In most mines, brown diamonds account for 15% of production. The brown color makes them less attractive to some people as gemstones, and most are used for industrial purposes. However, improved marketing programs, especially in Australia and the United States, have resulted in brown diamonds becoming valued as gemstones and even referred to as chocolate diamonds. [1]

Contents

A significant portion of the output of Australian diamond mines is brown stones. There are several causes for the brown color, including irradiation treatment, nickel impurities and lattice defects associated with plastic deformation; the last is the predominant cause, especially in pure diamonds. A high-pressure high-temperature treatment can heal lattice defects and convert brown diamonds into yellow or even colorless stones.

Rise in popularity

In 1986, massive amounts of brown diamonds were brought to the market. Rio Tinto opened the Argyle mine in Australia, in which 80% of the diamonds mined were brown in color. DeBeers followed this trend and in 1996 introduced massive number of brown diamonds to the market. These brown diamonds were previously used only for industrial purposes.

The value of brown diamonds was hard to determine since the GIA and other third party assessors did not evaluate these diamonds. To rectify the issue, Rio Tinto created a grading system where C1 represents the lightest color and C7 the darkest.

In 2000, the fine jewelry company Le Vian trademarked the term "chocolate diamond" and introduced a new brown jewelry line. Le Vian worked with the supplier Rio Tinto and partnered with the retailer, Signet Jewelers. The brand heavily advertised the 'chocolate diamond' line including a massive TV media spend. The campaign was a success; in 2007 virtually no one was searching for "chocolate diamonds", and in 2014 the number jumped to 400,000 times a year.

Occurrence

Diamonds occur in various colors, including blue, yellow, green, orange, various shades of pink and red, brown, gray and black. Before the development of the Argyle diamond mine in Australia in 1986, most brown diamonds were considered worthless for jewelry; they were not even assessed on the diamond color scale, and were predominantly used for industrial purposes. However, marketing strategies changed in the 1980s and brown diamonds have become popular gems. [2] [3] The change was mostly due to supply: the Argyle mine, with its 35 million carats (7,000 kg) of diamonds per year, makes about one third of global production of natural diamonds; [4] 80% of Argyle diamonds are brown. [5] The percentage of brown diamonds is lower in other mines, but it is almost always a significant part of the total production. [6] Consequently, scientific research has intensified on causes of brown color in diamond and ways to alter it.

Notable brown diamonds

Causes of color

Irradiation

Pure diamonds, before and after irradiation and annealing. Clockwise from left bottom: 1) Initial (2x2 mm) 2-4) Irradiated by different doses of 2-MeV electrons 5-6) Irradiated by different doses and annealed at 800 degC. Irrdiamond.jpg
Pure diamonds, before and after irradiation and annealing. Clockwise from left bottom: 1) Initial (2×2 mm) 2–4) Irradiated by different doses of 2-MeV electrons 5–6) Irradiated by different doses and annealed at 800 °C.

Irradiation of diamond by high-energy particles (electrons, ions, neutrons or gamma rays) produces vacancies in the diamond lattice by ejecting carbon atoms. Those vacancies produce green color centers in pure transparent diamond and yellow-green color in yellow diamonds. The color of yellow diamonds results from small numbers of nitrogen atoms replacing carbon in the lattice. Heating the irradiated diamonds to temperatures above 600 °C results in brown color associated with aggregation of the vacancies, with or without nitrogen involved. [16]

Such irradiation and annealing treatment can occur in nature because diamonds are often accompanied by uranium-containing ores which emit alpha particles. However, the thus produced color is restricted to a thin surface layer of few micrometers. [17] Homogeneous color can be produced if the treatment is performed artificially, using electrons, neutrons or gamma-rays. Radiation treatment induces characteristic sharp optical absorption lines which can be easily detected by spectroscopic techniques. [16]

Brown synthetic diamonds

Synthetic diamonds created by compressing graphite to several gigapascals and heating to temperatures above 1500 °C are usually rich in nitrogen. Nitrogen in those diamonds is dispersed through the lattice as single atoms and induces yellow color. Nickel is often added to graphite to accelerate its conversion into diamond. Incorporation of nickel and nitrogen into diamond induces brown color. Nickel is easily detectable by characteristic, sharp optical absorption and luminescence signals making such diamonds easily identifiable. [18]

Natural brown diamonds

Whereas the brown color due to irradiation or nickel impurity can be easily recognized through spectroscopic (e.g. absorption) measurements, the majority of natural brown diamonds do not show any characteristic absorption peaks. Whereas the consensus has been reached that the color relates to the plastic deformation, the particular reason has been reliably identified (large clusters of vacancies) only in type IIa natural brown diamond. [19] Other recent results suggest that these large clusters of vacancies (mini-voids) are a likely cause in other types of diamond as well. [20] [21] [22] Those lattice defects are most likely responsible for the color of the notable diamonds described above.

Heat-treated brown diamonds

The concept that brown color might be related to lattice imperfections has led to a technique to convert brown diamonds into more valued light-yellow or even colorless ones: the diamond is subjected to high pressures of 6–10 GPa and temperatures above 1600 °C that heals (anneals) those defects. [1] The technique has been demonstrated in several research laboratories in Russia and the United States. In March 1999, Pegasus Overseas Ltd (POL) from Antwerp, Belgium, a subsidiary of Lazare Kaplan International, started marketing such diamonds that were processed by General Electric (GE). Those diamonds therefore received the name GE POL (or GEPOL) and were marketed in the US as Bellataire diamonds. The existence and identity of the treatment process was considered so important that micrometer-sized letters "GEPOL" were inscribed with a laser on the girdles of every treated diamond. [23] In 2004, however, the GE diamond section was purchased by Littlejohn & Co. and renamed Diamond Innovations. Since 1999, several companies around the world have adopted the technique and use various brand names for the processed diamonds. [24]

See also

Related Research Articles

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<span class="mw-page-title-main">Gemstone</span> Piece of mineral crystal used to make jewelry

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References

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  6. ↑ Deines, P; Harris, J.W.; Gurney, J.J. (1997). "Carbon isotope ratios, nitrogen content and aggregation state, and inclusion chemistry of diamonds from Jwaneng, Botswana". Geochimica et Cosmochimica Acta. 61 (18): 3993–4005. Bibcode:1997GeCoA..61.3993D. doi:10.1016/S0016-7037(97)00199-3.
  7. ↑ "The Golden Jubilee". Archived from the original on 2011-05-05. Retrieved 2009-08-03.
  8. ↑ "The Earth Star – Famous Diamond". attributed to; Famous Diamonds by Ian Balfour and Diamonds – Famous, Notable and Unique by GIA. Diamond Articles. Archived from the original on 2009-07-08. Retrieved 2009-08-03.
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  15. ↑ "Lot 430. Property of a lady of title – The Lesotho I diamond, Harry Winston" . Retrieved 2009-08-04.[ permanent dead link ]
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  17. ↑ Kaneko, K.; Lang, A.R. (1993). "CL and optical micro-topographic studies of Argyle diamonds". Ind. Diam. Rev. 6: 334.
  18. ↑ Kanda, H. (2000). "Large diamonds grown at high pressure conditions" (PDF). Brazilian Journal of Physics. 30 (3): 482–489. Bibcode:2000BrJPh..30..482K. doi: 10.1590/S0103-97332000000300003 . S2CID   121445098.
  19. ↑ Mäki, Jussi-Matti.; Tuomisto, F; Kelly, C J; Fisher, D; Martineau, P M (2009). "Properties of optically active vacancy clusters in type IIa diamond". Journal of Physics: Condensed Matter. 21 (36): 364216. Bibcode:2009JPCM...21J4216M. doi:10.1088/0953-8984/21/36/364216. PMID   21832322.
  20. ↑ Collins, A.T.; Connor, A.; Ly, C-H.; Shareef, A.; Spear, P.M. (2005). "High-temperature annealing of optical centers in type-I diamond". Journal of Applied Physics. 97 (8): 083517–083517–10. Bibcode:2005JAP....97h3517C. doi:10.1063/1.1866501.
  21. ↑ Jones, R. (2009). "Dislocations, vacancies and the brown colour of CVD and natural diamond". Diamond and Related Materials. 18 (5–8): 820–826. Bibcode:2009DRM....18..820J. doi:10.1016/j.diamond.2008.11.027.
  22. ↑ Hounsome, L. S.; Jones, R.; Martineau, P.; Fisher, D.; Shaw, M.; Briddon, P.; Öberg, S. (2006). "Origin of brown coloration in diamond". Physical Review B. 73 (12): 125203. Bibcode:2006PhRvB..73l5203H. doi:10.1103/PhysRevB.73.125203.
  23. ↑ Read, Peter G. (2005). Gemmology. Butterworth-Heinemann. p. 162. ISBN   978-0-7506-6449-3.
  24. ↑ O'Donoghue, Michael (2006). Gems. Butterworth-Heinemann. p. 102. ISBN   978-0-7506-5856-0.
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