How does the colour of a fancy coloured cultivated diamond come about?

With the global market for lab diamonds already exceeding US$2 billion and expected to reach US$5 billion by 2023, lab diamonds have a huge market potential. Lab-grown diamonds, like natural diamonds, can come in a variety of beautiful colours. So do you know how these colours are created?

Yellow lab diamonds

The HTHP (High Temperature High Pressure) method and the CVD (Chemical Vapour Deposition) method are the two main types of lab diamonds, the HTHP method produces a yellow range of diamonds due to the fact that the synthetic diamonds produced by this method do not form the N3 colour centre of natural diamonds due to their rapid growth rate. The yellow tint is formed.

When it comes to Ib diamonds, it is important to mention the 'canary' yellow diamond, which is part of the vivid yellow colour of Ib diamonds, the vividness of which depends on the amount of N atoms.

Brown lab diamonds

The majority of synthetic brown diamonds are CVD diamonds, which, with the exception of a few that are almost colourless, have a certain brownish tinge to them. CVD diamonds are Type IIa diamonds due to the virtual absence of N in the synthesis conditions.

The cause of the colour of natural brown diamonds is also related to internal plastic deformation, but natural brown diamonds can be either Type I or Type II.

Colourless lab diamonds

Colourless synthetic diamonds can be synthesised by either the HTHP or CVD method. As HTHP diamonds often contain N, which is the cause of their yellow colour, N absorbers must be added to obtain a colourless synthetic diamond, and as HTHP synthesised colourless diamonds contain almost no N, they are Type IIa.

Alternatively, high temperature and pressure treatments can be used to transport and concentrate isolated N in the crystal structure to produce Type Ia diamonds, which are also known as GE-POL diamonds. Only high purity CVD synthetic diamonds are free of brownish tones and ultimately colourless.

Blue lab diamonds

Natural blue diamonds are mostly Type IIb, meaning that they have little to no N (nitrogen) inside, but some amount of B (boron). Therefore, synthetic diamonds can be made blue by using the appropriate method to turn them into Type IIb diamonds.

For example, the HTHP method can be used to produce Type IIb diamonds by adding an N absorber and the appropriate amount of B, or the CVD method can be used to produce Type IIa diamonds by adding the appropriate amount of B.

Green Grown Diamonds

Natural green diamonds are the result of GR1 absorption due to the natural radiation received by the diamond. The same principle applies to synthetic diamonds that wish to turn green, which can be obtained by placing them in artificial irradiation facilities such as cyclotrons, electron irradiation treatments, neutron irradiation treatments, etc.

It is also possible to add the HTHP method of synthetic diamonds directly to element B to obtain a combination of Ib and IIb synthetic diamonds, which can also be combined to obtain a green synthetic diamond as Ib is yellow and IIb is blue.

Red lab diamonds

Synthetic diamonds can also produce the precious red range. Red synthetic diamonds can be synthesised using the HTHP method with type Ib diamonds, which are first irradiated to obtain blue and blue-green, and then heated to give a final pink or purplish red diamond.

This type of pink synthetic diamond often has absorption lines of 637nm and 575nm, mainly due to N-V (hole + lone nitrogen).