A synthetic diamond is also called synthetic diamond, laboratory diamond or cultured diamond.

The synthetic diamond is a diamond manufactured in the laboratory by man. It has the same physical and chemical characteristics as natural diamond composition, hardness, gloss, shine, etc. It is therefore not a fake diamond but from a diamond which is not natural.

A short history

It was in 1954 that General ElectricMore specifically Tracy Hallpublishes its research and findings in the magazine Nature. It was at this time that the creation of the first synthetic diamond was officially recognised^.

The first real use of synthetic diamonds in jewellery (gem quality) began in the mid-1990s. But it was not until the late 2010s that these synthetic stones began to gain ground in jewellery.

Properties

Diamonds, when they reach a certain degree of purity, combine exceptional mechanical, optical, thermal and electronic properties. Synthetic diamonds can be artificially “doped” with boronat the phosphorus or thenitrogenThe introduction of defects in the crystal structure (e.g., the formation of a new crystal) can be used to modify the electronic properties of the material. The introduction of defects in the crystal structure ( nitrogen-lacune centres or NV, for example) is also used in quantum physics.

A diamond can be single crystal, or polycrystalline, i.e. made of many smaller crystals. Large, clear, single-crystal diamonds are typically used in jewellery.

Hardness

Synthetic diamond is the hardest material known. Hardness is defined as scratch resistance and is rated from 1 (softest) to 10 (hardest), according to the Mohs mineral hardness scale.

The hardness of a synthetic diamond depends on its purity, crystal perfection and orientation: the hardness is higher for pure and flawless crystals

Impurities and inclusions

Each diamond contains atoms other than carbon in concentrations detectable by analytical methods. These atoms can be grouped into macroscopic phases called inclusions. Impurities are generally avoided, but may be incorporated, as a means of controlling certain properties of the diamond. 

For example, a pure diamond is an electrical insulator, but a diamond with boron added is a conductor, and in some cases superconductor^. 

Synthesis technique

The HPHT technique (in French High pressure, high temperature) consists in mixing carbon (in an abundant form) and transition metals (which will act as catalysts) and subjecting the whole to a very high pressure (about 58,000 atmospheres) and temperature (about 1400°C). The diamond is then formed by germination and growth. In the temperature gradient method, a diamond seed is inserted into the reactor before the reaction.

The CVD technique (Chemical vapor deposition; in French Chemical Vapour Deposition) consists of growing the diamond in successive layers. The method consists of placing a layer of diamond (substrate or seed) in a chamber with a pressure of typically one tenth of an atmosphere. From thehydrogen and the methane (precursor gases) are then injected and the whole is then ionised using a microwave (frequency 2.45 GHz). A plasma is then initiated and the resulting species (ions, radicals, etc. from the initially injected gases) adsorb onto the substrate. The formation of a diamond layer that grows with time takes place after diffusion and surface reaction of the reactive species.

Difference with natural diamonds

 Differentiating between synthetic and natural diamonds is difficult and may require special equipment.

The natural diamond giant De Beers is developing several techniques to detect these new diamonds. One of these techniques is to detect the growth form of the diamond, which is not the same as in nature. For example, diamonds obtained by the HPHT method create cube-shaped growth sectors. Some forms of impurities are also not found in nature.

The CVD method, on the other hand, produces diamonds that are more difficult to differentiate from natural diamonds, as they are very pure and the impurities and cubic growth zones are less distinguishable.

Certification

 A synthetic diamond is evaluated according to four criteria, exactly as for the diamond natural: the 4 Cs: Carat (the weight of the diamond), Color (the colour of the diamond), Clarity (the purity of diamond) and Cut (the size of the diamond).

The main gemological laboratories that issue certificates for natural diamonds also issue certificates for synthetic diamonds, using exactly the same criteria to define their characteristics. The only difference between the two certificates is that on one it says “Natural Diamond” and on the other “Laboratory Grown Diamond”.