Most “gem quality” diamonds - like those used by Nightingale - are grown using a process known as chemical vapour deposition (CVD).
Here is the process in more detail...
1: Hydrocarbon gas and hydrogen are mixed at high temperatures in a low-pressure vacuum chamber. Some molecular hydrogen is converted into atomic hydrogen.
2: A small substrate or “seed” of diamond is introduced into the gas mixture at a temperature between 900 and 1200°C.
3: Hydrogen-carbon species travels across the surface of the “seed” until it finds an available carbon atom to attach itself to, increasing the thickness of the seeded growth by one atom.
4: Process repeats itself endlessly, with the diamond growing three dimensionally while maintaining its original crystal structure.
5: When the diamond created reaches its optimum size, it is cut and polished in the same way as a naturally occurring diamond.