Brown Diamonds

Brown Diamonds are the most common color variety of natural diamonds. The brown color makes them less attractive as gemstones and most are used for industrial purposes, however, technical advances and improved marketing programs, especially in Australia, have resulted in brown diamonds becoming valued as gemstones in recent years. A significant portion of the output of Australian diamonds 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 prodominant 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.



Causes of Color Diamonds
Irradiation 
Irradiation of diamonds by high energy particles produces vacancies in the diamond lattic by ejecting carbon atom. 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 number of nitrogen atoms replacing carbon in the attince. Heating the irradiated diamonds to temperatures above 600 'C result in brown color associated with aggregation of the vacancies, with or without nitrogen involved.
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 mictometers. Homogeneous color can be produced if the treatment is performed artificially, using  electros, neutrons or gamma rays. Radiation treatment induces characteristic sharp optical absorption lines which can be easily detected by spectroscopic techniques.

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 in 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

Natural brown diamonds
Whereas the brown color due to irradiation or nickel impurity can be easily recognized through spectroscopic measurements the majority of natural brown diamonds do not show any characteristic absorption peaks. Whereas the consensus has been reached that the color reates to the plastic deformation, the paricular reason has been reliably identified only in type lla natural brown diamond. Other recent results suggest that these large clusters of vacancies are a likely cause in other types of diamonds as we. Those lattice defects are most likely responsible for the color of  the notable diamonds.

Heat-treated brown diamonds
The comcept that brown color might be related to lattice imperfections has led to a technuque to convert brown diamonds into more valued light-yellow or even colorless ones. the diamonds is subjected to high pressures of 6-10 GPa and temperatures above 1600 'C that heals those defects. The technique has been demonstrated in several research laboratories  in Russia and the United States. In March 1999, Pegasus Overseas Ltd from Antwerp, Belgium, a subsidiary of Lazare Kaplan International, 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 diamonds in 2004 however the GE diamonds 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 processe diamonds.