Dating back to the 19th century, there have been many claims for creating a diamond in a lab. For many years, the conventional wisdom was that diamond would form only under conditions of high pressure and high temperature (HPHT). Indeed, the first commercially available “CVD” diamond was produced at high pressure and high temperature by General Electric in 1956.
In 1954, two years before the announcement of HPHT synthesis, a patent was issued for another type of diamond growth: the CVD (chemical vapor deposition) process. Early reports of gem-quality CVD diamond were greeted with doubt and distrust and not verified until many years later. In the late 1980s, scientists with a highly advanced understanding of the technique discovered how to reproducibly grow diamond using the CVD process.
The CVD process is quite different from the natural diamond formation. It produces a diamond from a heated mixture of a hydrocarbon gas (typically methane) and hydrogen in a vacuum chamber at very low pressures. Under normal circumstances, heating this mixture at such low pressures would produce graphite or some other non-diamond form of carbon. But in a CVD growth chamber, some of the hydrogens is converted to atomic hydrogen, which promotes diamond formation since diamond is more stable in this environment. The conversion of molecular hydrogen to atomic hydrogen is accomplished through methods such as the application of microwave energy, an electric discharge, or hot filaments. When atomic hydrogen is present in the gas phase, two chemical processes occur:
- Graphite and other non-diamond carbon react with the atomic hydrogen and evaporate in a newly formed gas phase.
- Atomic hydrogen reacts with the original hydrocarbon gas (methane) to form a highly reactive carbon-hydrogen species. When this species decomposes, it gives up its hydrogen to form pure carbon: diamond.
To produce a gem-quality CVD diamond, a diamond seed crystal (natural, HPHT, or CVD in origin) is introduced into the gas mixture, at an elevated temperature of 900 to 1200°C. The activated carbon-hydrogen species travels across the surface of the diamond seed until it finds an available carbon atom, and then attaches itself to this seed atom. The seeded growth is now one carbon atom thicker. This process repeats itself endlessly to replicate the crystal structure of the diamond seed crystal in three dimensions.
To date, diamond crystals 3 to 5 mm thick have been grown and fabricated into gemstones. One of the subtleties of seeded CVDgrowth is that even though it can produce thick growth vertically, lateral growth is limited by quality considerations. Thus the seed diameter must be at least as large as the desired diameter of the final diamond. Each new diamond growth requires a seed crystal, so either the seed must be reclaimed and reused, or growth capacity must be set aside to continuously manufacture seeds.
CVD diamonds are cut and polished in the same manner as natural diamonds, at the same cost. The stones can be laser inscribed on the girdle to allow easy preliminary identification.