September - November 2005

December - January 2006

Wilawan Atichat 1 Boontawee Sriprasert 1' 2

Research Team:
Wilawan Atichat 1, Boontawee Sriprasert 1' 2, Thidakorn Putharat 2, Thitinsee Pawaro 1, Patra Srithunayothin 1, Asst. Prof. Dr. Pornsawat Wathanakul 1' 3, Assoc. Prof. Dr. Wisuth Pisuthi-Anont 1' 4, Archara Saeng-Ariyavanich 5, and Areeratt jfCornduangkaew' 5

1. Gem and Jewelry Institute of Thailand
2. Department of Mineral Resources
3. Kasetsart University
4. Chulalongkorn University
5. The Office of Atomic Energy for Peace

Beyond nature : Challenges ahead

Diamond is a special gem, which is very popular and highly expensive due to its scarcity, beauty, and durability. Diamond can be highly valued though it is small in size. According to these factors, diamond has been made and known as precious object for a long time. Generally, clarity, color, cut, and carat or quality indicate value or price of diamond. Apart from satisfaction, beauty, and quality of diamond, the most important factor for diamond value is whether it is natural or not. In diamond trading markets, its value and price mainly correlate with quality and beauty. Anyhow, if the natural occurrence of diamond is concerned together with those factors, its value and price will be increase definitely. If it is so, there will be no or less problems. However, the problem which may arise in the near future is how to test the natural occurrence of diamond. In this modern world, research and new developments in technology are highly progressed. Sooner or later, the difference between natural and unnatural diamonds or other gems might be beyond the ability of common people to identify.

As for the easy explanation, but it might be difficult in practice, on how to distinguish real diamond from fake diamond is the natural or man-made occurrence. Natural diamonds must originate from nature, wherever sources or whichever countries they are from. Only through the appropriate cutting process that make diamond shine its glittering spark for the eyes of man. For unnatural diamonds, we can divided them into three categories: (1) diamonds, which are synthesized in the laboratory; (2) treated natural diamonds; and (3) diamond simulants which look like or are similar to natural diamonds but not their structure and composition.

It is found since the year 2000 that the first two categories of unnatural diamonds have played a pivotal role and increased fright in diamond trading markets. Treated diamonds, such as irradiated diamonds, HPHT diamonds, and synthetic diamonds from well-known manufacturers were sold increasingly in gems markets. The consumers might have not been well informed or they could not make the identification. Those unnatural diamonds are produced not only in the Unites States by General Electric, Novatek, Chatham, Gemesis, Apollo, Lucent, and by Sumitomo in Japan but also in Sweden, China, Russia, India, and Korea. The testing method to identify whether it is the natural diamond or treated diamond is now become highly complex and requires advanced technology.

At present, the testing for diamond if it is natural or unnatural needs "sufficient information from the analysis in various aspects including specific physical, optical, and chemical characteristic data, together with data obtained from advanced tools such as UV-Visible-Near Infra-red Spectrometer and Fourier Transform Infra-red Spectrometer, etc. The author and researchers have studied on natural diamonds, treated diamonds, and synthetic diamonds for some times in order to investigate and accumulate testing techniques for separating natural from unnatural occurrence and color of diamonds. Therefore, we would like to disseminate information on diamonds and itsbeyonei-nature status, WnicYi Vs challenging to us all in the near future.

Treated diamond: similar to natural occurrence

Diamond treatment is for at least two aims. Firstly, to make diamond more beautiful(clear, colorless, sparkling, or color enhancement). Secondly, to purify diamond. Sometimes, there might be disputes between sellers and buyers who are not fully informed of the treated diamonds in questions. The common diamond treatment process and frequently found in gems markets is to make diamond cleaner and clearer. For instances, filling with colorless substance in the crack or fracture, and applying laser for eliminating the inclusion. The popular processes to change diamond's color for commercial purposes are coating by synthetic thin film diamond; irradiation with various techniques which change to blue, green, brown, orange, yellow or black; annealing after irradiation to produce green, brown, orange, pink, red, and purple; HPHT treatment to produce colorless, yellow, and greenish yellow; and hybrid techniques to produce pink, red to purple diamonds.

Most of the natural diamonds (approximately 98 %) are type la (Bruton, 1978; Field, 1992; Nassau, 1994) which contain about 5,000 atoms in 1 million atoms (approximately 5,000 ppm) of nitrogen as trace element. They also have the structural defects at 4'15 and 340 nanometer, which caused by aggregation replacement form of 3 atoms of Nitrogen. While type Ib, Ila, and lib diamonds have Nitrogen in very small proportion (40,20, and less type la diamonds are always in greenish yellow to yellow tint color. Type la diamond treatment which includes brown diamonds treated with HPHT Process can be changed to greenish yellow ( Reinitz et al., 2000., Collins etal., 2000; and Hainschwang et al, 2005). The hybrid process including irradiation, annealing, HPHT, and low temperature low pressure annealing can change color of diamonds to pink, purplish red and orange brown (Wang et al., 2005).

There is about 1 % of type Ib diamond from the natural diamonds in the world. This type of diamond has single nitrogen atom as impurity, and commonly found in yellow diamonds and synthetic diamonds.Type Ila diamond is rarely found in the nature and it posses very little Nitrogen. By HPHT treatment, it could be changed to colorless or white diamond (Moses, et al., 1999; Smith, et al., 2000; and Fisher and Spits, 2000) or the brownish pink could be changed to pink diamond (Hall and Moses, 2000).

Type lib diamond comprises of Boron element and is very rare. It is often found in light blue to dark blue colors. The brownish blue diamond could be changed to light blue color (Hall and Moses, 2000).

Diamond treated by irradiation

Color in the diamond occurs from two causes: (1) impurity atoms; and (2) defects in diamond structure (caused by natural radiation or 'irradiation 'tecnrirqire\). Tin^t "two causes will form the color centers in diamonds and effect the ability of transparency of white light spectrum in the diamond. The daylight, in fact, comprises of seven visible colors that are violet, indigo, blue, green, yellow, orange, and red (the visible range of light is 400-700 nanometer; the invisible range is infra-red spectrum, which is more than 700 nanometer and ultra-violet spectrum, which is less than 400 nanometer). When the white light shed on diamond, the color centers(defect and impurity atoms) will absorb some spectrums of the light and emit other spectrums, which make us see the color of the diamond. In short, color of diamond results from the visible light emitted or absorbed by impurity atoms and structural defects in the diamond. The colorless diamond results from the color centers emit all color spectrums in the light through the diamond.

Diamond treatment by electron, neutron, gamma and alpha particles irradiation will cause structural defects in diamonds since these high power rays could move the carbon atoms and push them into the interstitial position. The absorption occurring with the vacancies in neutral status will result in the absorption at 741 nanometer or GR1 Center(General radiation); the shorter wavelength absorption such as GR2 to GR8 at 430 to 413 nanometer; and the absorption at 393 nanometer or ND1 Center (Negative vacancy).

These kinds of absorption always cause the diamond to change its color to green or greenish blue color. The examples of irradiated diamonds found in the market are in green, blue, pink, red, and black colors as feature in Figure 1.

The author and researchers conducted the experiments on irradiated diamonds by neutron and electron irradiation in order to investigate special characteristics as well as accurate and timely testing method to distinguish irradiated colored diamonds from natural colored diamonds.

Neutron irradiation from the nuclear research reactor 1 at the Office of Atomic Energy for Peace could be classified into two groups.

1 .Diamond irradiation at the external tube (CA" 3) by the nuclear research reactor 1, the first improvement, at the Office of Atomic Energy for Peace when there is epithermal neutron flux about 3.0 x 10 U neutron per cm" 2 per second for 36 hours. The neutron fluences are about 3.89 x 10 16 neutron per cm" 2.

2. Diamond irradiation at the internal tube(CT) by the nuclear research reactor 1, the first improvement, at the Office of Atomic Energy for Peace when there is epithermal neutron flux about 2.8 x 10 13 neutron per cm" 2 per second for 12 hours. The neutron fluences are about 1.21 x 10 18 neutron per cm" 2. Most of the samples for neutron irradiation are type la diamonds; laA, laB, and laAB. They fluoresce under long wave ultra-violet in various colors such as light chalky blue, green, and yellow.

Most of the neutron irradiated diamonds are green with different tones as feature in Figure 2 '

As for the electron irradiation, it is conducted by the electron accelerator kindly provided by IBA company in Rayong Province. On the treatment, the electron irradiation dose about 120,000 kGy or 12,000 Mrad is used. The samples for electron irradiation are yellowish white diamond; brown diamond; neutron irradiated diamond; and unidentified irradiated diamond. The samples turn into green in different color tones as feature in Figure 3.

The experiment on neutron and/or electron treated diamonds embedded in graphite rod and annealed in the electric furnace at 650 degree Celsius for 5 hours was conducted. By using this method, die green neutron and electron treated diamonds turned into brownish yellow. The brownish red unidentified irradiated diamond turned into dark red diamond as feature in Figure 4.

Figure 4. The green neutron and/or electron treated diamond turned into brownish yellow and the brownish red unidentified irradiated diamond turned into vivid red diamond after annealing.

The reason why the color of irradiated diamonds has changed after annealing treatment is because there had been the structural defects in diamonds with nitrogen from electron irradiation treatment. The irradiation will cause the vacancies, which are stable at the room temperature. According to the annealing treatment, these vacancies will move and will be caught by nitrogen in different forms in order to form the new color centers. As for type Ib diamonds, the vacancies caught by single nitrogen, will form the color center with the absorption at 1.945 eV (637 nm, N-V), which will turn into pink or red. On type la diamonds, the vacancies caught by nitrogen in A or B aggregate, will form the color center with the absorption at 2.463 eV (503 nm, N-V) or namely H3, center, ami 1.499 e-V (495 ?M «m namely H4 center. The absorption at H3 or H4 center will turn the diamonds into yellow, orange, or brown depending on its original color prior to the irradiation and the intense of the absorption caused by the defects from irradiation and annealing treatments.

The reason why the color of irradiated diamonds has changed into green or yellow after annealing treatment is because the nitrogen might be at atomic vacancies, among interstitial or as aggregation of nitrogen or single nitrogen. When neutron or electron irradiation enter into diamond, they will cause vacancies or defects inside it. This kind of irradiation treatment will show the spectrum at 393 (ND1), 595, 741 (GR1) nm. If annealing these irradiated diamonds, the vacancies will move and penetrate into their atomic structure. The moving vacancies will be caught by other defective structures(usually by nitrogen). This will cause the complex nitrogen vacancy combination of color center. These irradiated and annealed diamonds will show the absorption of UV-VIS NIR at 595, 503, and 496 nm; the absorption of Mid Infra-red at 4940 (Hlb)and5170(Hlc)cm- 1.

It is found that high pressure high temperature treated diamond or HPHT diamond can be seen in every color such as colorless, yellow, greenish yellow, yellow-green, pink, and blue. The majority of HPHT diamonds with the GE POL trademark produced by Pegasus Overseas Limited (POL) and General Electric Company(GE) Laboratory are natural type lla diamonds and mostly weigh less than 2 carats. They mostly appear in fancy cuts(oval, marquise, and pear) and almost in coloiless ot nearly coloiless(D-G). It is likely that they are of the clarity grade range from IF or VVS1 to VVS2, VS1, and VS2 for the rest. Looking inside, these diamonds have graining and a little bit blemishes. Under the Polariscope, the twisted cross (Hour-glass like), the stripe or spot arrangement and gray, blue and orange strain colors, are found. Most of them do not fluoresce under ultra-violet light and have Raman peak at 3760 cm- 1.

Apart from HPHT colorless diamonds from GE POL, the Nova Diamond, the company manufacturing of HPHT colored diamonds, has produced HPHT green as well as yellow diamonds from brown diamonds and launched into the market. The diamonds being used by Nova Diamond for HPHT treatment are type la diamonds, which comprise of nitrogen impurity atoms and they are 98 % of the world's naturally found diamonds. As a result, it is more difficult and more challenging to identify HPHT diamonds from natural diamonds.

The test of colorless diamonds and fancy colored diamonds whether they are natural occurrence or not is one of the hardest tasks. Since there is no HPHT facilities in Thailand, the experiment on HPHT diamonds could not be conducted. Therefore, the author and researchers have to find HPHT diamonds to study and compare so as to search for testing techniques. Fortunately, the manufacturer kindly provided samples of HPHT diamonds .tor. this, stiujy which are in white, yellow, and orange colors as shown in Figure 5.