Gem diamonds

Several superstructures and defect superstructures based on sphalerite and on wurtzite have been described. The tI16-FeCuS2 (chalcopyrite) type structure (tetragonal, a = 525 pm, c = 1032 pm, c/a = 1.966), for instance, is a superstructure of sphalerite in which the two metals adopt ordered positions. The superstructure cell corresponds to two sphalerite cells stacked in the c direction. The cfla ratio is nearly 1. The oP16-BeSiN2 type structure is another example which similarly corresponds to the wurtzite-type structure. The degenerate structures of sphalerite and wurtzite (when, for instance, both Zn and S are replaced by C) correspond to the previously described cF8-diamond-type structure and, respectively, to the hP4-hexagonal diamond or lonsdaleite, which is very rare compared with the cubic, more common, gem diamond. The unit cell dimensions of lonsdaleite (prepared at 13 GPa and 1000°C) are a = 252 pm, c = 412 pm, c/a = 1.635 (compare with ZnS wurtzite). 

To be referred to next is the most modem diamond anvU type, generating pressures of order of 10 -100 GPa. The cell illustrated in Figure 6(e) consists of two gem diamonds with optically flat surfaces, between which a sample confined in a drilled hole of a thin metal gasket is sandwiched. To attain isostatic compression an inert gas or an organic liquid, like a 4 1 volume mixture of methanol and ethanol, is contained with the sample. The generated pressure is measured directly from the pressure shift of the fluorescence line of mby powder mixed with the sample. Temperatures to 5000 K can be obtained by laser heating. The quantity of sample confined in a typically 0.1-mm-wide hole is extremely small, just a few microcrystals. At present, research has focused on in situ observations using X-ray and other optical methods, rather... 

Fortuitously, just as India s supply of diamond was running out, deposits were discovered in Brazil and just as Brazil s were nearly exhausted, huge supplies were found in Africa. Today the African continent is the major supplier of natural diamond to the world. Most gem diamonds come from the coastal deposits in South West Africa whereas Zaire and Angola mines yield industrial quality diamonds. Sizable... 

Of the natural raw diamonds produced in 1990, 45% were further processed to industrial diamonds and 55% to gem diamonds. This 55% of gem diamonds accounts for 98% of the total value of natural diamonds, whereas industrial diamonds only account for 2%. 

Without the high regard, that gem diamonds enjoy in our society, the extraction of natural diamonds would be completely uneconomic. Even in the rich Kimberley seams in South Africa, only 2.5 carats (0.5 g) of diamonds are obtained per t of processed rock, only 0.4 carats of diamond being obtained in 1985 in the Premier Mine (also in South Africa). 

The alluvial deposits on the cost of Namibia together with the adjacent Namaqualand to the South is the richest gem diamond source in the World. 

The exceptionally high hardness of diamond, the easy cleavage along the three main lattice planes of the crystal, its high refractive index and its optical dispersion makes diamonds a valuable raw material for brilliants. The gem diamonds utilized are still exclusively natural diamonds. 

The quality of gem diamonds is judged by the 4 C-method, comprising color, clarity (purity size and kind of inclusions), cut (the quality of it), and carat (weight). 1 carat equals 0.2 g. The measure originates from antiquity-it is defined to be the mass of a seed from the carob tree. These tend to weigh exactly 0.2 g with surprising reproducibihty. 

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