Lab grown diamonds have been around since the 1950's, but engineering has only recently become advanced enough to produce perfect diamonds -- specimen that rival anything that was ever pulled from the earth. In fact, we're so good at manufacturing diamonds these days, some growers are actually going to the trouble of putting defects back into them on purpose, so that they further mimic their earth-mined sisters! Yes... one major criticism of lab grown diamonds is actually that they are too perfect.
There are two main processes labs use to grow diamonds: CVD (Chemical Vapor Deposition) and HPHT (High Pressure-High Temperature). We're admittedly not engineers or scientists over here, but we'll do our best to break those down a little bit more in highly simplified terms:
The CVD process
These diamonds begin as a thin slice of diamond called a 'seed'. The original diamond 'seed' is often a tiny bit of HPHT produced diamond. This 'seed' is placed inside of a special sealed and pressurized chamber and heated to extreme temperatures -- around 800 Celsius.
The chamber the 'seed' starts in is also filled with a carbon rich-gas. This gas ionizes into plasma after being exposed to technology similar to what is used in microwaves or lasers. The ionization process breaks down molecular bonds in the gases, causing pure carbon to become available. This pure carbon then adheres and crystallizes itself to the 'seed'...ultimately creating a rough diamond.
The HPHT process
This was the first method of producing diamonds. These diamonds are made using one of three press manufacturing processes, and also require a small diamond 'seed' as an origin to grow from. The three slightly different press processes consist of a belt press, the cubic press, and the split-sphere (BARS) press. Each one of these three press processes involve creating an environment of simultaneous extreme pressure through physical force (using an upper and lower anvil to literally squeeze the materials) and temperature.
The diamond 'seed' is placed directly into carbon and then exposed to temperatures of around 1500 degrees Celsius alongside extreme pressure (around 1.5 million pounds per square inch, to be exact). The pure carbon melts under this heat and then starts to form a diamond around the starter 'seed'.
In either method, growing diamonds typically requires less than a month for most sizes. CVD-grown diamonds do often require additional post-growth treatments such as heat or irradiation to enhance or change their colors.
One interesting thing to note about grown diamonds is that the shape produced -- or 'rough' that comes out of the process -- does vary. This chart from GIA illustrates the different diamond shapes that are produced using the two lab growing methods as well as a natural diamond. Once a diamond is cut or faceted, this telltale feature is of course gone, leaving the only way to distinguish between the two is using extremely sophisticated electronic tools developed by gem grading labs.
Amazing, right?! Lab grown diamonds are truly marvels of modern engineering!