It is said that diamonds are a girl's best friend. But does that mean artificial diamonds are false friends? Not really, for they are quite useful to humans for many reasons.
In 1772, Antoine Lavoisier focused the rays of the sun on a diamond. It burned to form carbon dioxide, proving the diamond was nothing but carbon. From that time on, people have dreamt of making a big fortune by converting different forms of carbon - like coal and graphite - into diamond.
Diamonds are made naturally in the earth. Deep inside the earth, the pressure and temperature are both very high. Because of this, carbon (leftover from the remains of prehistoric life) is compressed into the regular crystal form of diamond. Because it is very compressed, diamond becomes very hard. In fact, it is the hardest substance on earth. It is also very, very rare.
The first claims
In 1893, French chemist Henri Moissan (and before him, James Hannay in 1879) claimed he had made an artificial diamond. He heated charcoal with iron to 3500oC in a furnace, till the iron melted. This was then suddenly plunged into cold water. As the iron froze, it would generate such high pressures that the carbon would be turned to diamond.
Many scientists tried to do what Moissan had done, but without success. Sir Charles Parsons, a famous British engineer who had invented the steam turbine, tried out Moissan's method again and again, for forty years. In the end he got frustrated and wrote a paper saying that Moissan's method was impossible. What Moissan and others might have got was spinel (a mineral very similar to diamond), and not diamond.
What we often call 'American diamonds' are not the same as artificial diamonds. They are actually crystals of a mineral called zirconia.
The GE diamond project
General Electric (GE), a major American company, tried to make diamonds in the 1950s. On December 16th, 1954 the first 'real' artificial diamond was made. GE scientist Tracy Hall had created a machine that could apply up to 18 gigapascals of pressure, and be heated up to 2000oC. Diamonds made this way are called HPHT diamonds.
Hall's method involves dissolving graphite in molten iron in a compression chamber. The chamber is then pressed between two (or four) enormous anvils, and heated till diamonds start to form. Diamonds made this way are very tiny (no more than 0.15 mm across). You can't make jewels out of them, but they are very useful for making diamond cutters and industrial abrasives (remember diamond is the hardest thing on earth).
Imagine you are carrying a bag with 18 million tonnes of earth on your shoulder, inside the world's hottest volcano. That's the kind of pressure and temperature you need to make artificial diamonds. (And that's how natural diamonds are made anyway).
Crystal vapour deposition
In the 1980s, a new method was developed that didn't need high pressures and temperatures. In this method, a hydrocarbon gas (like methane) is converted into plasma, and then sprayed onto a tiny seed crystal. The carbon ions in the plasma get deposited on the seed crystal, and bind to it. This way the crystal 'grows', until you get a suitably large diamond. Such diamonds are called CVD diamonds.
This can be used to make diamonds big enough for making jewellery.