Color of Gemstones

A milder version of this color, pink has been known to work wonders. This
colour too relaxes muscles, relieves tension and has an overall soothing
effect, so much so that jewellers suggest that pink gemstones shoul be worn
at any important event.

While a gemstone can change the personality and health of one who wears it,
any individual can in turn change the character of the gem over time. Sweat
and body oils give turquoise a green tinge and darken it. On the other hand,
emeralds may absorb oil and get more lustrous. Rubbing against the body,
pearls may gradually lose their shine and opals, their fire. Some stones
like amber develop an electric charge when rubbed while hematite acquires
magnetic properties. Whatever effects a gem may have because of its colour,
three factors determine the value of a coloured stone. The hue defines
whether it is red, blue or green. The next factor is the purity of the
colour or the absence of a mixture o hues. Finally, the intensity of colour
is another criterion. The value of a stone depends on these three
parameters, the best tone being the most expensive. For instance, a good
ruby should shine bright and pure red, without a trace of blue, while a blue
sapphire must be a deep blue without a trace of yellow or black. The most
expensive diamond would have no colour at all, or else should have a deep
colour, the richest and purest in nature.

The perception of colour also depends very much on the light that falls on
the object. A white sheet appear red when viewed in red light. Textiles too
appear to change colour with the ambient light and customers always look at
fabric in sunlight outside the store when different pieces have to be
matched. Similarly, the colour of a gemstone too depends very much on the
light that falls on it. It will appear very different when viewed by
candlelight, in sunlight or in the glare of a fluorescent lamp. Jewellers
and gemmologists therefore take the sun's rays to be several colours.
Electric lamps whose light spectrum is as close to sunlight as possible are
necessary for gem grading.

Another problem faced by gem buyers is that colour perception varies
considerably with each individual. Indeed some humans and all dogs are
colour-blind. Sensitivity to colour too varies as a result of the internal
structure of the eye. The human retina consists of rods and cones. The cones
are of three types, those sensitive to red and yellow, other to green and
yet others to the blue to violet regions of the light spectrum. The
sensitivity of these differs from person to person and therefore colour
judgment can be very subjective;this can affect the estimated price of a
gemstone significantly. Although one jeweller may sell in good faith a ruby
that appears to him as a pure red and so of high value, a second valuer may
see a trace of blue in the same stone and thus declar it to have a much
lower value.

Colour appears to affect well-being and behaviour. It affects man in a
combination of biological, physiological, psychological, social and cultural
factors. For instance, magenta, red, orange, yellow, and yellow-green are
warm colours associated with excitement, happiness and comfort. Cool colours
include violet, blue, light blue, cyan and sea green and induce calm abd
peace as well as sadness, withdrawal and repression. Scientific studies show
that red light is more likely to produce epileptic seizures than blue light.
Red also increases blood pressure, respiration and heart rate. Naturally,
people connected the lustre and the colour of gemstones with medicine,
astrology and emotions. Even today, in an age of scientific cynicism, men
and women wear gems for their mystical ability to endow health, wealth and
happiness. The belief that gems possess magical properties has led to a huge
volume of oral traditions and literature, apart from billions of dolloars
worth of trade and industry. To increase profits, jewellers have devised
various processes than can improve or even change the colour of a gemstone.
These are described in a later chapter.

Business Oriented Practcal Gem Training Classes

Gem Classes by Lion Dr. S.P. Karunanayake.

• Practical Gem Identification, Valuation & Gemology
• Field Trips - Purchasing Gems at Gem Mining Areas.
• Heat Treatment and Cutting & Polishing of Gems.
• Basic Jewelry Essentials.
• Import & Export Procedures - Invoicing and Documentation.

Other Services:

• Gem Cutting & Polishing
• Heat Treatment of Sapphires.
• Jewelry Making Orders Undertaken.

For more information please visit http://www.uslankagem.com

Gemstones Enhancement in Sri Lanka

The trade of gemstones is phenomenally large, the annual turnover being more than US$50 billion. A major share of this market is in diamonds, as these stones are bought for investment as well as for decorative purposes. The price of a diamond, however, has no relation to the actual cost of production, but lies in its comparative rarity. Over the years, by controlling the quantity of diamonds entering the market, the diamond trading company has kept the price of good quality diamonds stable and at a high level, ensuring its investment value. Sales publicity by jewellers, too, has encouraged a preference for diamonds over that for other gems. Substitutes like cubic zirconia, moissanite and Yttrium Aluminium Garnet (YAG), though equally lustrous and attractive, do not have the same appeal.

Most gemstones, diamonds included, are pale in color, often with inclusions, cracks or voids and since such stones are commonly available, they are cheap and so difficult to sell to discerning buyers. The improvement of the color and the clarity of these low quality stones is therefore of extreme importance to the trade. Research in material science has now given us the ways to improve the appearance of gemstones permanently and there is indeed considerable enhancement in the value of gemstones after treatment. On the other hand, there are people who have voiced serious objections to altering what nature has provided. It is a fact though that man is constantly improving on nature to make profits for himself, whether it is plastic surgery or gemstone enhancement.

There are many techniques used to enhance the color and quality of gemstones. Radiation treatment using gamma rays, electrons, x-rays are few. The most famous method of gemstone treatment is heat treatment. Specially corundums are heat treated to produce bright color blue sapphires. Heat treatment is a very popular method for enhancing colors of sapphires. Heat treatment to standards are accepted by the trade.

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.

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.

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.

Enhancement of gemstones – Part I

Luster or fire, color or lack of color, iridescence, opalescence, asterism and chatoyance (cat’s-eye or star effect) are some of the unique properties that increase the beauty and value of a gemstone. Since the beginning of civilization jewelers have increased their profits by using various means of making gemstones appear more attractive and valuable. In Harappa and Lothal, sites of the Indus valley civilization, archaeologists have uncovered pieces of agate and carnelian that were cooked in charcoal furnaces to change their color and enhance their appeal. Even in the ancient cities of Egypt, Ur, and Babylon, gem traders commonly practiced some kind of gemstone enhancements. Egyptian payrii describe some of their techniques, as for instance, how pearls were bleached by passing them through the intestines of a live cock or by soaking them in slaked lime. Verdigris and vinegar, oil and calves bile formed part of a process to make Emeralds. Various gemstones of Indian origin found in the tomb of Tutankhamen indicate that Indian traders of that time were experts at improving gems. Practitioners of the art of treating gem kept their treatment recipes, even those based on deception, as very closely guarded secrets then, as they do even today.

Colored stones, as all stones other than diamonds are termed, account for about a fifth of the huge gemstone trade. The supply as well as the price in the colored stone market is dictated by demand, availability and ethnic considerations, all driven by greed, fashion and illusion. Large clear specimens of good colors are exceedingly rare and cannot be valued by any formula; their price is determined entirely by negotiation. It is therefore only natural that even today, improved, simulated or imitation gems continue to defraud customers. The gems that stud jewelry are sometimes backed with metal foil, cloth, butterfly wings, and peacock feathers to increase their luster. Doublets and triplets where a genuine but thin stone is glued to cheaper material to increase the weight and luster are common, especially in Opals. Synthetic emeralds are grown over natural quartz to simulate a large and much more valuable emerald. To uncover such fakes, a recognized laboratory should carry out tests to detect such other fraudulent methods.