How do they form?

HPHT diamond (High-Pressure, high-temperature)

HPHT is a traditional diamond cultivation method. This method uses special equipment to simulate the high-pressure and high-temperature environment (about 50,000 atmospheric pressure, 1300-1600°C), which similar to the earth condition that natural diamonds are formed.

The growth process of HPHT diamonds:

  1. Diamond seed crystals are placed under the growing chamber, which is heated to 1300-1600°C and the pressure is higher than 50,000 atmospheres.
  2. Molten metal (elements such as iron, nickel, cobalt) breaks down the high-purity carbon source.
  3. Carbon atoms are then deposited onto the diamond seed crystal and crystallised into diamonds. This crystallisation process takes days to weeks to form one or more crystals.

In the past, it is a difficult process to produce colourless diamonds using HPHT , which they contain nitrogen, which causes a yellow colour. However, with the advancement of technology, the colour of HPHT-grown diamonds have been improved. HPHT method can also be used to change the colour of natural diamonds to colorless, pink, green, blue or yellow.

D, E and F colour diamonds can now be obtained using HPHT method, but diamond colour below H can still be resulted from immature laboratory.

In terms of clarity, most HPHT diamonds are VS to SI grades. Higher clarity colorless diamonds require longer growth times, and the growing temperature and pressure also need to be more precisely controlled.

 

CVD diamond (Chemical vapor deposition)

HPHT diamonds mentioned above requires slow growing time, and hence in 1960, scientists developed a better diamond cultivation method named "chemical vapor deposition (CVD)", to compensate the disadvantages of HPHT.

The growth process of CVD diamond:

  1. Place the diamond seed crystal in a vacuum growing chamber, eliminating all impurities.
  2. Inject methane and hydrogen into the growth chamber .
  3. Heat the growth chamber to 900-1200°C.
  4. Turn on high-power microwaves to separate the carbon atoms from the hydrogen atoms.
  5. Free carbon atoms gradually attach to the diamond seed crystal and crystallise into diamonds  

Most CVD-grown diamonds are initially brown, but nearly colorless or pale yellow crystals can also be produced. HPHT is frequently used to improve the colour of CVD diamonds to the colorless range. "Colorless" CVD diamonds that are chosen to be cut into gemstones tend to have higher clarity than natural diamonds, mostly between VVS2 and VS1.