Differences Between Natural and Lab Grown Diamonds
A diamond is a mineral that is one of the polymorphic modifications of a chemical element such as carbon (C). Thus, diamond-like graphite and lonsdaleite are completely composed of carbon atoms. The difference lies only in the crystal lattice of these minerals (in the arrangement of atoms). The distinguishing features of diamonds are that it crystallizes in the cubic syngony at very high temperatures and pressures (for example, 1600°C and 6 GPa). A lab grown diamond is the same diamond, only grown in laboratory conditions. It has the same crystal lattice, identical physical and chemical properties, and it crystallizes naturally in the cubic system. A brilliant is a polish diamond, either natural or lab grown. Strictly speaking, a brilliant denotes the type of the cut of a diamond that is designed to bring out the optimum optical properties of a diamond.
This is not the case with imitations of a diamond. These are other minerals and chemical compounds which are fundamentally different in chemical composition, crystal lattice, and physical properties. For example, moissanite is silicon carbide (SiC), and it has few similarities with a diamond. It is used as an imitation because of its diamond-like optical properties (dispersion, brilliance, and refractive index).
|Mineral, compound||Chemical formula||Refractive index||Specific weight||Syngony||Mohs scale of hardness|
|Lab grown diamond||C||2.42||3.53||Cubic||10|
As we found out, natural and lab grown diamonds are one and the same in terms of material composition. But then the question arises how can they be distinguished from each other, and what is the difference between them, if any difference even exists.
The answer to this question lies in the process of diamond crystal growth in nature and in the laboratory. In nature, diamonds crystallized deep in the mantle (at depths of ≈ 150 km) hundreds of millions, or, in some cases, even several billion years ago. Further, this diamond was there for some time (millions of years), and then it was abruptly brought to the earth's surface during the formation of Kimberlite explosion pipes (Figure 1). In deposits, diamond is found in rocks of the ultrabasic composition of alkaline orientation called "Kimberlite." It is very difficult to establish the duration of the growth process of a natural crystal, especially since there are crystals that have gone through several periods of growth.
In the laboratory, diamond crystallizes in two ways: the first way is in high-pressure cubic presses - HPHT method (Figure 1). The crystal grows on the seed by diffusion of carbon atoms on it (graphite is used as a source) in the melting of the catalyst metal. Here, temperature and pressure conditions are created that are close to those in which natural diamonds have grown. The duration of the process of synthesis of gem-quality crystals is 7-14 days. The second approach is in chemical deposition chambers - CVD method (Figure 1). The crystal grows from the gas phase (methane + hydrogen) on a substrate under plasma conditions. In this method, ultra-high pressures are not needed, as crystal growth is due to the chemical reaction of deposition on the diamond substrate. The synthesis process with a low quality of the crystal can last only one day.
This is where the differences between natural and lab grown diamonds exist. The differences between natural and lab grown diamonds (especially diamonds) can often be identified only with the help of instrumental research methods in qualified laboratories. Below we will consider two main differences between natural and lab grown diamonds that exist today.
The shape of the crystals. This would be a very reliable and useful hallmark if the diamond came to you before the cutting/polishing process. In nature, the vast majority of diamond crystals are found in the form of flat-faced octahedrons or forms of dissolution of octahedrons. HPHT diamonds have a much richer morphology, where, in addition to the faces of the octahedron, there are faces of the cube, rhombic dodecahedron, trigontrioctahedron, etc. CVD diamonds are almost always grown on <100> substrates, which gives them the shape of a cube.Figure 2. Morphology of natural and lab grown diamond crystals.
Inclusions. This sign is also very important and reliable. It can be detected in both crystals and diamonds. As we have already discussed, in nature, a diamond crystallizes in the mantle. Under such conditions, it captures inclusions of other minerals that crystallize in the same environment. These are minerals of the garnet, olivine, diopside groups, and various sulfides (Figure 2). In the laboratory, the diamond is in the metal melt (HPHT) and then captures inclusions of metal carbides (Figure 2). Or, it is under plasma deposition (CVD) conditions and captures other (non-diamond) carbon phases (Figure 2).Figure 3. Inclusions in natural and lab grown diamonds.
Thus, the two principal distinguishing features of natural and lab grown diamonds are the shape of the crystal and inclusions. Further, taking these main features into account, several additional features are determined that allow the diagnosis of the origin of the stone.
However, when the crystal is already cut and polish and has the highest clarity characteristics, a new question arises. If we do not see either the shape of the crystal or inclusions, then how is it possible to diagnose a diamond? The answer is that these differences can only be registered in a laboratory or with an experienced gemologist. They consist in the presence of impurities in the structure at the atomic level. Also, the most important feature is the zoning of the internal structure, which was formed in the process of growth (it determines the shape of the crystal). These features of the diagnosis of natural and lab grown diamond crystals (diamonds) will be considered in the following articles.
Author: LGDeal Gemology Department