Brown diamonds

A collection of brown diamonds in the Smithsonian. 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. The brown color makes them less attractive as gemstones because of the reduced glimmer, 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 in recent years and even referred to as chocolate diamonds.[1] A significant portion of the output of Australian diamond mines is brown stones. A large amount of scientific research has gone into understanding the origin of the brown color. Several causes have been identified, including irradiation treatment, nickel impurities and lattice defects associated with plastic deformation; the latter is considered as the predominant cause, especially in pure diamonds. A high-pressure high-temperature treatment has been developed that heals lattice defects and converts brown diamonds into yellow or even colorless stones.

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 even not 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 (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

References

  1. 1 2 Collins, A; Kanda, Hisao; Kitawaki, Hiroshi (2000). "Colour changes produced in natural brown diamonds by high-pressure, high-temperature treatment". Diamond and Related Materials 9 (2): 113. Bibcode:2000DRM.....9..113C. doi:10.1016/S0925-9635(00)00249-1.
  2. Harlow, George E. (1998). The nature of diamonds. Cambridge University Press. p. 34. ISBN 0-521-62935-7.
  3. Kogel, Jessica Elzea (2006). Industrial minerals & rocks. Society for Mining, Metallurgy, and Exploration (U.S.). p. 416. ISBN 0-87335-233-5.
  4. "The Australian Diamond Industry". Retrieved 2009-08-04.
  5. Erlich, Edward; Dan Hausel, W (November 2002). Diamond deposits: origin, exploration, and history of discovery. p. 158. ISBN 978-0-87335-213-0.
  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. Bibcode:1997GeCoA..61.3993D. doi:10.1016/S0016-7037(97)00199-3.
  7. "The Golden Jubilee". 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. Retrieved 2009-08-03.
  9. "Earth Star Diamond". Internet Stones.com. 2006. Retrieved 2009-08-04.
  10. "Star of the South Diamond-Famous Diamonds". Retrieved 2009-08-04.
  11. Dickinson, Joan Y. (2001). The Book of Diamonds. Courier Dover Publications. p. 108. ISBN 0-486-41816-2.
  12. 1 2 "Incomparable Diamond". Retrieved 2009-08-03.
  13. 1 2 "The World of Famous Diamonds and Other Gems". Retrieved 2009-08-03.
  14. Hesse, Rayner W. (2007). Jewelrymaking through history. Greenwood Publishing Group. p. 68. ISBN 0-313-33507-9.
  15. "Lot 430. Property of a lady of title – The Lesotho I diamond, Harry Winston". Retrieved 2009-08-04.
  16. 1 2 Walker, J. (1979). "Optical absorption and luminescence in diamond". Rep. Prog. Phys. 42 (10): 1605. Bibcode:1979RPPh...42.1605W. doi:10.1088/0034-4885/42/10/001.
  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". Brazilian Journal of Physics 30 (3): 482. Bibcode:2000BrJPh..30..482K. doi:10.1590/S0103-97332000000300003.
  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.
  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. 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. 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 0-7506-6449-5.
  24. O'Donoghue, Michael (2006). Gems. Butterworth-Heinemann. p. 102. ISBN 0-7506-5856-8.
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