Tuesday, March 10, 2009

Improving Diamonds - Part IV

Diamonds must be heated only in vacuum or else will burn or turn into graphite. Even at low temperatures of 300C to 400C, cracks and other defects that were hidden within the stone appear. Above 1500C, diamond turns to graphite even in vacuum unless it is kept under very high pressures.

Natural pink diamonds are rare, expensive and in demand. Recently it was reported that selected light brown diamonds do sometimes turn pink after irradiation and heating and this led to a scramble to make pink diamonds. Soon afterwards, large quantities of pink diamonds were available in the market; these looked better than their naturally colored counterparts.

A note of caution must be sounded in regard to the treatment of diamonds. The ultimate color that a diamond takes on after irradiation and heating depends entirely on the impurity atoms and their arrangements within the stone. Apparently, if a diamond lacks nitrogen within it, it can be given a pink color. Rough diamonds from various sources are mixed before they come to be cut in India and Sri Lanka. As a result, it is near impossible to predict that color a diamond will finally achieve. Experienced traders claims to be able to select the proper stones for treatment but they are not always successful

One thing must be made very clear. Under no circumstances can an off-color diamond ever be made whiter or colorless! The original color of a diamond, whether it is yellow or brown, cannot be removed by any simple means. One can merely add another color to mask the original shades. The new high pressure, high temperature (HPHT) techniques described later, however, claims to bleach brown diamonds to some extent, but this process it extremely expensive and probably useful only for large stones. Though undesirable colors can be masked by treatment, the clarity of the stone cannot be improved. Those faults are repaired by laser drilling and fracture filling, as described earlier.

Natural diamonds are much more expensive than artificially colored ones. A customer who buys a fancy colored natural diamond has to be assured that he is not being cheated. Some kind of certification by a gem testing laboratory is therefore necessary. In the USA and Belgium, any treatment applied to a diamond must be disclosed to the buyer. When a colored diamond of unknown origin comes into the market, tests have to be made to determine the origin of the color.

Once any induce radioactivity disappears below background levels, it becomes difficult to say if it has been treated. Perhaps electron spin resonance measurements (ESR) and optical absorption spectroscopy done on the stone when it is kept at -160C may determine the history of any treatment. These measurements are expensive and so it is not worth testing a diamond weighting less than 2 carats. The trade assumes that all small colored diamonds have been irradiated, unless proven otherwise.

These sophisticated measurements would then fix the value of the stone. Spectroscopic tests find that the treatment invariably produces as absorption line at a wavelength of 595nm, but heating the stone for several hours at 900C seems to destroy this line. Irradiated green diamonds almost always display a distinct absorption line at 741nm. Yellow treated stones absorbs at 640nm or at 595nm, the two lines accompanied by a 504nm line and sometimes one at 985nm. Only the relative intensity of these lines decides whether the color is natural or not. Therefore a stone with strong absorption lines at 595nm and at 496nm and probably another at 504nm is almost certainly treated, though some naturally colored diamonds show only the 504nm absorption line. However these criteria are not definite as some diamonds show contrary characteristics.

Monday, March 9, 2009

Improving Diamonds - Part III

Electron bombardment causes a change in the color of diamond as long as they energy of the electrons is enough to knock out atoms from their positions in the lattice. In addition, the energy must also be so high that the electrons penetrate more or less uniformly through the stone, or else the color is confined to the surface. Electrons from a high-energy accelerator knock out atoms to a position intermediate between other atoms, leaving a hole in the lattice. Such hole interstitial pairs are called point defects and they produce a light blue shade, reasonably similar to those very sought after Blue Jaeger. With increasing electron bombardment, this blue gets deeper as the irradiation increases the number of defects. At much higher electron energies, the carbon atom that is knocked out has enough energy to knock out other atoms and a cascade of displacements results. The consequent heavy damage imparts a green color to the crystal.

The most economical method to color a diamond quickly and permanently is to put it into a nuclear reactor inside which large numbers of high speed neutrons fly around. Neutrons do not carry any electric charge and so are not stopped by the nuclei of atoms in the crystal. Neutron irradiation therefore induces a uniform color in the diamond. A neutron being a heavy particle of weight two thousand times that of an electron is able to knock out carbon atoms even at relatively low energy. Again, a high energy neutron that collides with a carbon atom can give it enough momentum to hit other atoms and cause a major series of displacements. Depending on the length of neutron irradiation, the color can be regulated from a blue green tinge to emerald green to a very dark green and on to almost black. Badly flawed stones of poor color have thus been made a lustrous black by heavy neutron irradiation, hiding all the imperfections. The color and its intensity can be just right to offset the original unwanted shade and for this a certain artistic judgment is required.

There have been fears that anything put in to a nuclear reactor would turn dangerously radioactive. Diamond is pure carbon that does not become radioactive though trace impurities present in parts per million may cause some radioactivity. This activity dies down in a few days, but as a safety precaution, neutron treated diamond should be monitored carefully before release. Since slow neutrons are more effective in causing radioactivity, irradiation is best done in a fast reactor or a swimming pool reactor. Rumors that neutron irradiated black diamonds were passed off as natural stones and were a radiation hazards are groundless. In any case, some radioactivity is already present in the human body from the environment and from food and water; compared to this, the activity from colored diamonds is trivial. It is estimated that one has to wear jewelry with 3000 carats of treated diamonds to match the radioactivity already present in one’s body!

After irradiation, the induced color fades to some extent with the diamond is slowly heated at low temperatures of around 300C, but the produced color never completely disappears. If the stone is suddenly heated to a high temperature of about 900C, the defects produced by irradiation move around or migrate in the crystal and finally gather in clusters. Such defect clusters are associated with a rage of shades, from amber, orange and canary yellow to green, blue, red, or gold.

Friday, March 6, 2009

Improving Diamonds - Part II

When the atoms of carbon in diamond are arranged in a perfect formation, when the crystal lattice is perfect, the stone is colorless. This perfect diamond has a band gap of 5.6eV which can be bridged only by ultraviolet light. Such a perfect diamond therefore absorbs light only beyond the ultraviolet and these stones are colorless, and graded as pure white. More usually, impurity atoms of nitrogen, boron, manganese, or iron may be present in the concentration of a few parts per billion. These impurities are different in size from carbon and so distort the crystal lattice and cause defects levels within the band gap, leading to absorption in the visible regions. In contrast to colored stones like sapphire and ruby, the color is not related to the impurity, but to the defect in the crystal lattice arrangement. The diamond then appears in pale shades of yellow, brown, green or even blue. About seventy years ago, these pale blue diamonds were found in the Jaegersfontein mines in South Africa and are called Blue Jaegers. Slightly yellow and brown stones are the most common and therefore may be up to ten times cheaper than the best white stones.

In contrast to sapphire and ruby, impurity elements that cause color in diamond cannot be removed. Those who claim to bleach diamonds by heating with secret powders are deceiving the public.

Nuclear particles can knock carbon atoms of the diamond crystal lattice out of their place and thereby change the properties of the stone permanently. Irradiation of diamond by atomic particles results in a change in the color of the stone. The user of atomic radiation to improve diamonds has proved to be profitable. Poor color diamonds are not stable and remain with the dealers as dead stock. If these were treated to become novel colors like green, blue, pink or red and retailed with proper marketing and sales publicity, it could open up an entirely new market for jewelry. Soon after the discovery of radioactivity by Becquerel and Curies, scientists investigated the effects of newly discovered atomic rays on precious stones. Sir William Crookes found in 1904 that when exposed alpha rays from radium salts, diamonds turn green. These stones are housed in British Museum in London and their green color is evidence that the color is permanent. The diamonds are, however, radio active and even prolonged chemical washing did not remove the activity.

When the cyclotron was invented, diamonds were exposed to high energy beam from this machine. Dark green ‘cyclotroned’ diamonds resulted from the irradiation. But even at the high energies at which they emerge from the machine, the particles are stopped at a very shallow depth. The deep color is confined to near the surface and may be detected by the so called umbrella effect. Since the a great deal of research has been done on the irradiation of diamonds and other gemstones with beams of various particles and rays such as electromagnetic radiation (ultraviolet light, x-rays and gamma rays), electrons and heavier particles like neutrons, protons, and alpha particles.

Electromagnetic radiation has no effect on diamonds. X-rays and ultraviolet light shining on diamonds make some of them fluoresce, but there is no permanent change. Gamma rays do not color diamonds because they are not able to displace atoms in the diamond lattice, though it has been reported that they cause some increase in the luster. This could happen because there is a temporally shift in the electrons from one position in the crystal to another. Gentle warming or exposure to sunlight reverses any effect that is produced.

Improving Diamonds - Part I

The price of a diamond is determines by its physical properties and the rarer the stone, the more valuable it is. Most diamonds are somewhat yellow or brown in color and only about twenty per cent of rough stones from the mines yield flawless and color less diamonds. Pure white and flawless diamonds are therefore much in demand and expensive. Diamonds in deep shades of yellow, blue, green or even red are extremely rare in nature and so count as expensive collector’s items. Even rarer among these richly colored stones are the photochromic or chameleon diamonds that changes their color slowly on exposure to light. Obviously there is great profit if a diamond can be colored to a deep shade or made photochromic.

Jewelers have tried to enhance the color of gemstones for centuries by painting or dying the gem but tricks of this kind can easily be detected. As long ago as 1568AD, Benvenuto Cellini of Firenze dyed commonly available yellow diamonds with blue indigo to turn them green and sold as rarities.

The dye faded soon in time as all dyes do. In the last few years, a new process has been developed whereby a coat of colored synthetic diamond film is applied to the lower-girdle facets of a polished stone in order to simulate color in set diamonds. This is more stable than dying. The diamond film can withstand temperature of up to 600C and this is not affected even when the stones are heated to 600C during setting. However the coating will come off or be damaged when the diamond is re-cut or re-polished.

After cutting and polishing a diamond may exhibit cracks or metal inclusions that are seen as black or brown spots. Such stones are rejected, as they are supposed to bring bad luck. The trade has methods to remove these spots and hide the cracks and flaws. A highly focused laser beam is used to drill a very small hole up to the spot in the diamond. The stone is then boiled as acid to dissolve the inclusion. The tubular hole that reaches the surface is then filled with some colorless plastic or resin of high refractive index. The hole and filling are so small that a jeweler’s loupe cannot detect that the stone has been repaired. However, it is hardly worth doing this for small diamonds.

Microscopic examination of the filled stone can reveal that it has been doctored. When the treated diamond is tilted back and forth under a microscope, the reflected color sometimes changes from orange to blue and back to orange again in a flashing manner. The plastic filler also appears glassy, with an unnatural, melted look. Often air bubbles are trapped in the filler and near the surface there could be a cracked appearance.

The trouble with the plastic or resin filling is that in time; the plastic discolors, making the stone look worse than it did before the filling took place. A better but more difficult process forces lead glass of high refractive index into the hole, making a more permanent seal. Modern techniques using X-Rays can detect this lead glass filling as the filled areas appear opaque to x-rays. Another method for detecting this fracture filling is to examine the diamond by x-ray fluorescence, now commonly used to check gold purity. The lead in the glass filling signals it presence in the fluorescence detector.

Monday, March 2, 2009

Program to increase gem and jewellery exports in Sri Lanka

The National Gem and Jewellery Authority (NGJA) of Sri Lanka is exploiting new markets to restore the declining exports in the main markets of Sri Lanka. Acting Deputy Director General Ajith Perera said that the authority will launch market promotion programs in Russia, India and the Middle East soon.

As a result of the global economic crisis our exports have declined by 40-50 per cent compared to last year. The main markets for Sri Lankan gems and jewellery are the US and Europe and both markets are now in crisis, because at a crisis these are the products what consumers cut-off first from their shopping list, Perera said.

He said that all efforts will be taken to retain the existing markets as well as exploiting new markets. We encouraged out exporters to attend a big exhibition concluded on February 10 in the US and we gave cash incentives for 25 participants. The NGJA will open a Sri Lankan pavilion at the JCK Las Vegas show to be held from May 30 to June 02. Trade exhibitions have been organized in Russia, India and the Middle East.

We are planning to invite big buyers to the Facets 2009, the biggest annual gem and jewellery exhibition which will be held in September. National Gem and Jewellery Authority has also taken long-term measures to improve the quality of the products by extending training facilities, Perera added. The National Gem and Jewellery Authority will reimburse a part in the training fee and encourage skills development of the industry workers.

The industry sources saith that the impact of the crisis in the end market has spread to the bottom of the industry. There is a decline in mining due to drastic price reduction in the local market. The high gold price too has affected the gem and jewellery industry. Due to the crisis in the stocks and financial markets demand for gold has increased.

Sunday, March 1, 2009

Mangalabarana collection launches by Swarnamahal

Tradition look centre stage when to be brides were offered veritable selection of bridal requisites under one roof at a recently concluded Weddign Fair at Sirimavo Bandaranayake Memorial hall at BMICH.

Swarnamahal launched their ‘Mangalabarana 2009’ collection designed with the traditional bride in mind. Exquisite jewellery created in sovereign gold had matching eardrops and necklaces with a price range to fit the budgets of the average middle class consumer.

The yellow gold jewellery bridal package ranged from fifty thousand up to two hundred and fifty thousand rupees depending on the design and its weight in sovereign gold. The white gold Aura Collection studded with valuable diamonds was targeted at the more affluent. The prices ranged from approximately two hundred thousand up to eight hundred thousand rupees.

The brides were offered the option of purchasing jewellery designs off the counter at discounted rate at the exhibition or to have their bridal jewellery designed to suit their taste.

Expertly designed poruwa, settee back and other decorative flower arrangements were displayed by many wedding support services who offered every bridal requirement from Kiribath structures to Magul Bera, from Nilame costume to musical band. Bridal Salons offered competitive packages and discounts in products and services over the counter to brides accessing their services. From wedding cakes to cake boxes and cake structures to invitations the stalls had it all.

The exhibition had a good response from the couples who sought the convenience of shopping under one roof saving the hassle of running from pillar to post getting the requisites together for the celebrated moments.

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