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brown diamonds
Brown diamonds are the most common color variety of natural diamonds. The brown color once made 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. 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 are 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. 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;〔(【引用サイトリンク】url=http://www.costellos.com.au/diamonds/industry.html )〕 80% of Argyle diamonds are brown. The percentage of brown diamonds is lower in other mines, but it is almost always a significant part of the total production. Consequently, scientific research has intensified on causes of brown color in diamond and ways to alter it.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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