The principal crystallographic planes of diamond are the cubic (100), dodecahedron (011), and octahedron (111). The relative rates of growth on these planes are governed by the temperature and pressure conditions, together with the chemical environment during both growth and, in the case of natural diamond, possible dissolution during its travel to the earth’s surface. This, in turn, governs the diamond stone shape and morphology.
The phase diagram for diamond/graphite is shown in Figure 5.8.
3.4.4 Production of Synthetic Diamond
The direct conversion of graphite to diamond requires temperatures of 2,500 K and pressures of >100 Kbar. Creating these conditions was the first hurdle to producing man-made diamonds. The General Electric Company (GE) achieved this through the invention of a high-pressure/temperature gasket called the “belt” and announced the first synthesis of diamond in 1955. Somewhat to their surprise, it was then announced that a Swedish company, ASEA, had secretly made diamonds 2 years previously using a more complicated six-anvil press. ASEA had not announced the fact because they were seeking to make gems and did not consider the small brown stones they produced the culmination of their program! De Beers announced their ability to synthesize diamonds shortly after GE in 1958.
The key to manufacture was the discovery that a metal solvent such as nickel or cobalt could reduce the temperature and pressure requirements to manageable levels. Graphite has a higher solubility in nickel than diamond has; therefore, at the high-process temperatures and pressures the graphite dissolves in the molten nickel and diamond then precipitates out. The higher the temperatures, the faster is the precipitation rate and the greater the number of nucleation sites. The earliest diamonds were grown fast at high temperatures and had weak, angular shapes with a mosaic structure. This material was released by GE under the trade name RVG, for “Resin Vitrified Grinding” wheels. Most of the early patents on diamond synthesis have now expired and competition from emerging economies has driven down the price of this type of material to as little as $400/lb, although quality and consistency from these sources are still often sometimes questionable.