Post on 15-Mar-2018
transcript
Synthetic* diamonds, what is in a name? Presentation by Dr. Katrien De Corte, Chief Officer Education & Industry
The International Diamond Council defines synthetic diamonds as a “Man-‐made product that has the same chemical composition, crystal structure, optical and physical properties as a diamond.” Both natural and synthetic diamonds are made of carbon, have the same density and refractive index and share the same extra-‐ordinary hardness. The main difference is its point of origin: natural diamonds grow deep within the Earth and are mostly more than 1 billion years old, while synthetic diamonds are produced in a lab in a couple of days/weeks. Laboratory grown diamonds can be produced by two methods. One is HPHT, under high pressure and high temperature; the other is CVD, chemical vapour deposition. HPHT (High Pressure / High Temperature)
The purpose of this technique is to synthetically produce a diamond by replicating the formation conditions of diamonds deep in the earth. By applying high pressure (HP) and high temperature (HT) to a carbon source, one 'forges' a synthetic diamond. Swedish researchers at ASEA were the first to produce a diamond by using this method in 1953. They were soon followed by General Electric and De Beers. Since the seventies, gem-‐quality diamonds have been produced by HPHT synthesis.
CVD (Chemical Vapour Deposition) Chemical Vapour Deposition (CVD) is the process through which a diamond is produced from a (carbonaceous) gas. The principle was already known in the nineteen-‐thirties, but it took until the beginning of this century
before anyone succeeded in growing larger single diamond crystals instead of polycrystalline material. The gas (methane is usually used) is heated to a very high temperature. Atomic compounds are then broken down. Then, carbon atoms can be 'precipitated' onto a surface. Under the right conditions, a single crystal is formed that becomes thicker layer by layer. CVD diamonds are -‐ considering the complexity of the process -‐ usually light brown, making them relatively unattractive to the average diamond trader.
However, this brown colour can often be removed using HPHT treatment. Rough synthetic diamonds So far, both rough HPHT synthetic diamonds and rough CVD synthetic diamonds are easy to identify. HPHT synthetic diamonds grow in a typical cubo-‐octahedral shape while CVD synthetic diamonds have a characteristic plate structure. Polished synthetic diamonds The production of (near-‐) colourless gem-‐quality synthetic diamonds improved significantly the last years. Top-‐quality (D, LC) polished synthetic diamonds are on the market today. For most polished synthetic diamonds visual observation is not sufficient to identify the origin of the stones. Advanced technology and understanding of optical centra are key for identification. Important to note however is that all colourless synthetic diamonds on the market have no, or only minor amounts of nitrogen while more than 98% of colourless synthetic diamonds contain nitrogen in their crystal lattice. This significant difference in nitrogen concentration is an important tool to screen polished diamonds.
CONCLUSION In contrast to diamond imitations (e.g. moissanite, CZ) lab-‐grown stones have the same chemical and physical properties as natural diamonds. It is, therefore, in most cases impossible to distinguish polished lab-‐grown from natural diamonds by visual or microscopic examination.
Since full disclosure is fundamental to consumer confidence it is important to use alternative methods in order to separate the lab-‐grown from natural diamonds.
*For the purpose of this presentation the term “synthetic diamond” equals the terms “Laboratory-‐created”/ “Laboratory-‐grown”/ “Lab-‐grown”.