Wednesday, February 2, 2011

Gem Heat Treatment

There are other ways to enhance the beauty of a gem and make it more valuable. Irradiation is one such method.

All hot objects like the sun, the stars and burning coal radiate energy in the form of electromagnetic waves. Light waves are only a small small part of a wide spectrum of this radiant energy. The gamma rays that come out of radioactive substances are the most energetic of these waves and are used to cure cancers and sterilize food and other items. Less energetic are the X -rays that doctors use to internally examine the body. Lower in energy are the ultraviolet rays that help plants grow and human skin to acquire a dark tan. Even lower in energy are the infrared rays that we cannot see but whose warmth we can feel the warmth as our body absorbs them.

Our eyes are sensitive to rays in a narrow range of energies between the heat-giving infrared and the ultraviolet. These visible rays that we generally call "light" cover a spectrum of seven different colours, violet, indigo, blue, green, yellow, orange and red, a range of wavelengths that is commonly termed VIBGYOR and in this range, our eyes are most sensitive to the central region, green.

We call those objects that do not give off any radiation 'black' but usually this only means they do not emit visible light. If on the other hand they radiate light of all energies of the visible spectrum, they are usually termed 'white'. The sun whose surface temperature is around 6000C, radiates light with all these colours mixed in a certain proportion. This mixture appears white to the human eye. Artificial light has different ratios of the component colours and thus light from an incandescent bulb appears slightly red while a fluorescent tube produces a blue tinge.

So what makes gemstones so luminous and attractive? The beauty comes from the fact that they give out light waves that our eyes can see and to which our senses react. All objects reflect, absorb and emit light that falls on them. Objects like charcoal, that absorb all the light falling on them without any reflection, appear to be black. Conversely, a top quality diamond is colourless but reflects all the light that falls on it and so absolutely accurate only the paper of this book should really be termed 'white'. Between these extremes are the coloured gems that reflect light selectively. A good ruby for instance, absorbs all light but gives off light only in the red region. This is how we recognize and define colour.

A transparent medium bends a ray of light that enters it. The more it is bent the higher the refractive index of the medium. The refractive index of glass, for example, is around 1.33, while diamond has the highest index of 2.42. This is why a cut diamond glitters so much more than does glass or any other crystal. Sapphire has a refractive index of around 1.77, while zircon or Rangoon diamond clocks in at 1.98. Crystals of zirconium oxide have a refractive index of 2.18, the closest created so far to that of diamond. They are now grown commercially and sold as American diamond, cubic zircon or CZ, diamondite or diamlite. They are inexpensive but because of their high refractive index, are often mistaken intentionally or otherwise, for diamond.

Cutters make full use of the high refractive index of gemstones. When light rays coming out of the crystal hit the surface at small angeles they are sreflected back. They crystal can therefore be cut jwith plane flat facets so that any light that enters it exits only through the top surface or table. This enchances the brilliance of the stone and explains why diamonds are cut in that way.

Some stones have a crystal structure that splits a light ray that enters it into two rays of slightly different velocities and these travel in slightly different directions. no black spot viewed through such a crystal appears as two spots and this property of the crystal is called double refract but zircon does and this effect allows an immediate identification of the stone.

Yet another factor improves the appearance of a cut gemstones. The reflection of most clear gemstones varies with the colour of the light that that passes through it. This is 'dispersion' and causes different colours to flash outwards when the crystal is turned. This is known as the 'fire' of the gemstones. The refractive index and the dispersion as well as the stone determine the fire and light of the gemstone but the beauty of the gem increase with the perfection of the cut and the polish.

The total value of a gemstone, in general depends on several criteria. These are:

01. Rarity: It is the most important factor that determiners the value of a gem, in much the same way that an antique is most prized when it is the only one of its kind. The material itself might be very uncommon like the gajaratna. The rarity of a particular gem may also be due to its large size compared to most other gems of the same material as for instance the Black Prince's Ruby, or it's extremely fine or unusual colour as with the Hope Diamond. No doubt it is difficult to find buyers or sellers for such rare artifacts.

02. Clarity: Gems should be free of flaws, be they inclusions or cracks. In India, a buyer will not usually look at a diamond that has a flaw or black spot in it. This was not always the case and flawed stones were once considered attractive. Even now, some communities prefer to buy such diamonds because they believe that a genuine diamond always has flaws in it! Totally flawless or 'clean' in many species. Such perfection is almost unknown for example, in Colombian emerald and Burmese ruby.

03.Colour: Colour plays a big part in the determination of the value of a gemstone. The slightest tinge of colour, yellow or brown, in a diamond brings down its price to a fraction of the value it would have if it were totally colourless. Again a dark colour might be desirable in a green tourmaline, but if the colour is too dars the gem may actually lose value.

04. Transparency: A gem should be clear and not in the least milky. In nature, most rubies and sapphires are milky and these are often processed at the mine itself to remove this cloudiness.

05. Hardness: A soft gem will scratch easily and soon lose its polish and beauty from constant wear. Of course, soft gems such as amber and coral are in jewellery, but buyers are warned to handle them with great care. Pearls are also soft, but are able to retain their beauty for a long time if they are stored and cleaned properly.

06. Brilliance: Many gemstones vary in their refractive index so the light reflected from one stone could differ from that from another even if they are of the same family. proper cutting of the stone can optimise its lustre or brilliance.

07. Dispersion: As mentioned earlier, high dispersion improves the 'fire' of the stone. It often happens that if the stone is cut to give off most of the light, the ' fire' is decreased.

08. Beauty: The perception of beauty in a gem varies from one culture to another. The Chinese may desire green jade and the Europeans may buy coral and amber, but Indians on the whole prefer colourless diamonds. Tradition as well as the fashion of the time thus ultimately decides the worth of a gem.

09. Special Properties: Some stones and in particular sapphires, contain numerous thin, hair-like fibres of another mineral. This gives the stone a fibrous sheen like satin or silk and this is termed 'chatoyancy'. Generally, the stone is milky, but when the fibres are aligned, streaks of light appear as a cat's-eye effect. as in chrysoberyl. When the fibres are aligned in three crystal directions, a six-ray star effect results. The twelve-rayed star is most rare, and therefore the most valuable. Alexandrite as well as a few rare diamonds are photochromic, that is to say, they change colour when exposed to light.

As a result, techniques to produce cat's-eye and star effects in milky sapphires and rubies have been developed and have proved successful.

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