Article

Recently, a 0.43 carats yellow-coloured diamond was submitted for Grading at the Diamond Grading Laboratory of the Gemmological Institute of India (GII). The diamond was first sent to the Research Laboratory for analysis of properties and for confirming the authenticity of the stone. (Fig. 1)
Fig.1. Synthetic Diamond of 0.43 carats
Using the newly installed state-of-the-art equipments, which include FTIR (Fourier Transform Infra Red Spectrophotometer), DiamondView™ and DiamondSure™, the technicians identified the diamond as being a synthetic.

The DTC’s DiamondSure™ indicated that the stone be referred for further tests. The DiamondView™ exhibited the typical growth pattern and green fluorescence. Fig. 2 shows zoned fluorescence observed under the DiamondView™ for 0.43 carat Synthetic Diamond.



Fig. 2. Zoned Fluorescence observed under the DiamondView™ for 0.43 carat Synthetic Diamond.

The stone was then examined under the Thermo Nicolet 5700 FTIR fitted with Continuum Microscope and Linkham Heating-Cooling Stage. The FTIR exhibited a peak of 1344 cm-1 which is indicative of Type 1B Synthetic Diamonds (Fig. 3).



Fig. 3. Infra red spectrum of Synthetic Diamond of 0.43 carats

The stone was examined under 60x magnification, but it was quite cloudy and included. Synthetic diamonds which contain these clouds seem to be attracted to a magnet even when no large flux particles are present.

The diamond was attracted to a spherical magnet and could be suspended in mid-air. (Fig. 4). This suggests that the cloudy pinpoints might be at least partially composed of iron or iron-nickel flux.

Fig. 4. Synthetic Diamond 0.43 carats, being attracted by spherical magnet

What is a Synthetic?

The definition of Synthetic is material created in a laboratory using basically the same ingredients found in the natural products (Matlins and Bonanno, 1998). Synthetic diamonds have identical physical, chemical and optical properties as that of the natural diamond. Even though synthetic diamonds replicate the natural gem, they must be identified and prefaced with the word "synthetic," "created," or some origin indicator.

Why Synthetic Diamonds Are A Challenge?

The process by which the synthetic diamonds are produced and the ultimate identical appearance of the synthetic stones in almost any desired colour make it increasingly likely that gem quality synthetic diamonds will soon become available in the jewellery market in a large way. The properties of synthetic diamonds are so similar to the natural diamonds that their identification is a challenge to the gem industry.

Chemical Composition	Carbon
Hardness	10 on Mohs Scale
Lustre	Adamantine
Refractive Index	2.4175
Critical Angle	24.5°
Coefficient of Dispersion	0.044
Specific Gravity	3.52

Also none of the routine tests used for the separation of diamonds from its simulants is useful in distinguishing the synthetic diamonds. Even for any conventional gem testing laboratory, it is practically impossible to differentiate the synthetics from their natural counterparts. Therefore the most sophisticated instruments like FTIR, Laser Raman Spectrophotometer, DiamondView™, DiamondSure™ and UV/Vis/NIR Spectrophotometer, etc are absolutely necessary to identify synthetic diamonds. This is a serious challenge to the gem and jewellery industry as well as for most of the gem testing laboratories.

Diamond Formation in Nature and In Laboratory

Deep inside the earth, crystallization of carbon atoms into diamond takes place at very high pressure (70,000 kg / sq cm) and temperature 20,000o C. The temperature and pressure parameters are very important as crystallization of carbon atoms at lower temperature forms Graphite. Graphite and diamond are polymorphs of carbon. Graphite is the hexagonal structure of carbon atoms in flat hexagonal rings while diamond has cubic structure. Graphite under heat and pressure can be converted into diamond within few minutes whereas it takes at least a week to grow 5 mm gem quality synthetic diamond crystal.

It is theoretically possible to make a 2 carat diamond from the graphite in a pencil. Practically when this is tried out the synthetic diamond produced is of very small sized industrial quality. Graphite has a density of 2.27 gms / cc whereas that of diamond is 3.52 gms / cc. Hence for the same unit weight graphite occupies more volume. When graphite is converted into diamond there is a loss in volume which results in loss in pressure which ultimately decreases the speed of growth resulting in small sized diamond grit.

Graphite was regularly used as the starting material for the production of small synthetic diamonds. After many years of research, scientists came to the conclusion that when synthetic diamond grit instead of graphite was used as the source material, it was possible to produce small sized transparent gem quality diamonds.

Types Of Synthesis

Synthetic diamonds are grown by many different techniques. According to Dr. R.J. Wedlake (De Beers Diamond Research Laboratory, Johannesburg), they are:
• Growth from a molten solvent-catalyst.
• Shock wave synthesis.
• Direct transformation in the absence of a solvent.
• Growth from carbon melt.
• Meta-stable synthesis from a carbonaceous gas using a diamond seed crystal.

The process of diamond synthesis from carbon can be regarded as a three-stage process:

HPHT. To recreate the earth’s conditions of forming diamond at 150 – 200 kms deep, several HPHT methods have been developed. The two most important are the BELT system and its Russian counterpart – the BARS system. The incomplete science has now become a technology in which many processes are closely-kept secret. Both methods use a core reaction cell in which the diamonds are grown. This reaction cell is submitted to extreme pressures and temperatures. Inside the reaction cell, a carbon source is placed, as well as some seed crystals upon which the carbon gets precipitated during the process. Any HPHT device used for the creation of large synthetic crystals has to control a great number of process parameters (more than 500) in order to achieve sizable diamonds. A slow controlled growth is essential, with a long steady period of time, close to saturation point. Only a few minutes are needed to convert graphite to diamond, but it takes about three days to crystallise a gem-quality stone of 5 mm.

CVD. High pressure is normally essential for the formation of gem-quality diamonds, whether in the Earth’s interior or in the laboratory. However, growth of synthetic diamond by chemical vapour deposition (CVD) techniques, which does not require high pressure, is drawing increased attention worldwide. The CVD method is a revolutionary process, which is the exact opposite of HPHT. Instead of pressuring carbon into diamond, the carbon is freed to become pure diamond. Moderate heat is applied to a carbon cloud in a vacuum chamber causing diamond atoms to rain onto and grow a diamond seed. Methane and hydrogen gases are fed into a quartz tube about 50 mm in diameter, and a pump maintains the pressure in the tube to about 5% of atmospheric pressure. This mixture is heated by using a microwave beam. At temperatures of about 800°C, all atomic bondings are broken and the electrons are separated from the atomic nuclei. In other words, a plasma is created.
The freed carbon precipitates out of plasma cloud and is deposited on a substrate. Initially, small diamond crystals are randomly nucleated on the substrate; then, as the growth proceeds, these nuclei become larger and eventually coalesce to form a continuous polycrystalline film. The growth rate of this process is about 0.5 mm per day. Hydrogen gas is used to remove (etch away) all traces of non-diamond which is formed. Next to diamond, other forms of carbon are also formed, e.g. graphite, diamond-like-carbon and crossovers. The broken-down hydrogen gas is very reactive (radical) and eats away everything that is not diamond.

Features Of Synthetic Diamonds

Diamond crystals produced synthetically are often more regular than the natural ones. The irregularities of natural crystals could be due to the constriction by surrounding rocks when they grew and lack of carbon. Also natural diamonds may have been partly dissolved after their original formation. Typical characteristics of synthetic diamonds include:
• Cubo-octahedral shapes (a cubic modification of the octahedral growth that does not look natural).
• Metallic inclusions (iron or nickel) and paramagnetism.
• Clouds of pinpoints are seen in almost every synthetic, particularly the yellows.
• Fluorescence zoning in a cross-shaped pattern.

Gemmological Institute of India

The Gemmological Institute of India (GII) is India’s top-most authority on gemstones and diamonds. It is a non-profit making Public Charitable Trust, established, in 1971 for the promotion and development of Gemmology in India. It’s Research Laboratory – a first in the field of Gemmology in India – was started in 1976 and is recognized as “Scientific Industrial Research Organization” (SIRO) by Dept of Scientific & Industrial Research, Ministry of Science and Technology, Government of India.

The institute is the symbol of Integrity for the Indian Diamond Trade and Industry, being the only Trade-sponsored Diamond Grading Laboratory in Mumbai. It is a project of the Gem & Jewellery Export Promotion Council (GJEPC), supported by Bharat Diamond Bourse (BDB), Diamond Exporters’ Association Limited (DEAL) and Gem & Jewellery Exporters’ Association (GJEA).

To foster the growth of the Institute, GII has recently set up the National Research Centre for Diamonds and Gemstones. The Institute has imported latest Research equipment – DiamondView™, FTIR and Raman – for identification of synthetics. This has put GII on par with other international gemmological Research institutes. The move completes the vertical integration of the Institute’s services – Educational, Laboratory and Research.