Europe Fashion Jewellery
By Aamir Mannan.

In some minerals the figures very somewhat; thus in zircon, while the blue, white, and golden 'fried' type most popular in moderan jewellery are almost constant in their birefringence of 0.059. many of the ceyclonese zircons. particularly the green varieties, have a much lower double refractions, sinking in some cases practically to zero. this peculiaity is unique amongst minerals, and will be discussed at greater length in the chapter on zircon. synthetic rutile has been manufactured in the USA (since 1949), and its anormous double refraction will be one means of identification. calcite, though not a gemstone, plays an important part in gemmological instruments in its optocally pure form of 'ice land spar', how, apart from the refractometer, can one test whether a given stone doubly refractive or not? the simplest method of all, which with a little practice is quite easily carried out, is to examine the stone carefully 

with a powerful pocket lens-say giving 8x to 12x magnification. for example, look trougth the table facet of a zircon with a lens, and focus sharply on to the edges of the back facets where they adjoin the culet, and it will be noticed that instead of a single sharp line where the facets join. Colours plays an extermely important part in contributing to the beauty and popularity of precious stones. it was their colour that made such stones as turquoise and lapis lazuli among the first to attract the cupidity of early civilised man, and it is the mangnificent crimson red of ruby, the deep cornflower blue of the finest sapphires and the verdant green of emerald which (added to their transparency, hardness and rarity) have caused them always to rank amongst the supremely precious gems. only in the case of diamond, unique in this as in so many other ways, has a complete lack of colour been  regarded as the standard of perfection. those unversed in gemmology find it hard to realise that is most of the mineral species used as gems a wide colour range is possible, and that in their 'pure' state they would be

Indian kathak Fashion Jewellery.
By Aamir mannan.

COLOUR, COLOUR FILTERS, AND THE DICHROSCOPE

It is the presence of traces of these in an otherwise colourless crystal that produces their colour varieties. more will be said about the function of these 'transition' element , when dealing with the spectroscope. granted that there are a considerable number of red, blue and green gemstones, there are very few that can approach ruby, sapphire and emerald in their rich shades of these colour and a colour sensitive and trained eye can go a long way towards distinguishing them from other natural gemstones, glass imitations, or even synthetic counterparts. it is indeed worthwhile for a beginning

gemmologist to practice at every opportunity the sight identifications of the various gems and to pay special attention to the particular grades of colour typical of each species.If therefor in any two media which are in optical contact we can measure the angle where total reflection beings (i.e. the critical angle), and we known the refractive index of the denser medium. it is possible to calculate the refractive index of the rarer medium. this is the underlying principle of all total reflection refractometers (sometimes called total refractometers), but in the instruments designed for gem testing all calculation is ingeniously avoided. In such refractometer the optically dense medium of known refractive index is in the form of polished hemisphere of heavy lead glass, or a segment of such a

Indian And Uk Earrings Jewellery.
By Farazana aamir.

As With any other test, practice and perseverance are needed, and when the beginner has become accustomed to getting results in easy specimens he should strive to develop his skill to the limit and learn to gauge approximately the strength of the double refraction shown. the author has found this to be a most valuable accomplishment. the more thing that can be learned with a pocket lens the better-even if 

one possessed other apparatus this will not usually be available in an auction room or on other people's premises. this same test can, of course, be extended and rendered easier b using a microscope, which functions as a much more powerful lens, magnifications of 25 to 60 diameters being quite comfortable usable with stone held between the thumb and forefinger of the left hand, supported on the microscope stage. When even the most practised eye may find it difficult to distinguish between stone of difficult species but of very similar colour, simple instruments can sometimes be used to 'analyes' the colour in one way or another and make the separation simple. what we call 'white' light, i.e. light from the sun or other incandescent bodies, is composed of a mixture of all the colours of the rainbow. newton, in the year of the great fire of london (1666), was the first to show

 Fashion Earrings Jewellery. 
By Aamir mannan.
12.COLOUR, COLOUR FILTERS, AND THE DICHROSCOPE

When even the most practised eye may find it difficult to distinguish between stone of difficult species but of very similar colour, simple instruments can sometimes be used to 'analyes' the colour in one way or another and make the separation simple. what we call 'white' light, i.e. light from the sun or other incandescent bodies, is composed of a mixture of all the colours of the rainbow. newton, in the year of the great fire of london (1666), was the first to show that sunlight has a composite nature. he analysed a narrow beam of light that passed through a chink in the shutter into his darkened room, by the simple expedient of placing a glass prism in the path of the ray. light is variously refracted by a transparent solid according to its wavelenght, the red rays, of longer wavelength, beings less deviated than the shorter, violet, waves.

Jade Bridal Jewellery
By Aamir Mannan.
12.COLOUR, COLOUR FILTERS, AND THE DICHROSCOPE

When even the most practised eye may find it difficult to distinguish between stone of difficult species but of very similar colour, simple instruments can sometimes be used to 'analyes' the colour in one way or another and make the separation simple. what we call 'white' light, i.e. light from the sun or other incandescent bodies, is composed of a mixture of all the colours of the rainbow. newton, in the year of the great fire of london (1666), was the first to show that sunlight has a composite nature. 

he analysed a narrow beam of light that passed through a chink in the shutter into his darkened room, by the simple expedient of placing a glass prism in the path of the ray. light is variously refracted by a transparent solid according to its wavelenght, the red rays, of longer wavelength, beings less deviated than the shorter, violet, waves.

Blue Sapphire Party Jewellery.
By Aamir Mannan.
11.DOUBLE REFRACTION AND DISPERSION

As With any other test, practice and perseverance are needed, and when the beginner has become accustomed to getting results in easy specimens he should strive to develop his skill to the limit and learn to gauge approximately the strength of the double refraction shown. the author has found this to be a most valuable accomplishment. the more thing that can be learned with a pocket lens the better-even if one possessed other apparatus this will not usually be available in an auction room or on other people's premises. this same test can, of course, be extended and rendered easier b using a microscope, which functions as a much more powerful lens, magnifications of 25 to 60 diameters being quite comfortable usable with stone held between the thumb and 

forefinger of the left hand, supported on the microscope stage.

Blue Sapphire Bridal Jewellery
 By Aamir Mannan.
11.DOUBLE REFRACTION AND DISPERSION

As With any other test, practice and perseverance are needed, and when the beginner has become accustomed to getting results in easy specimens he should strive to develop his skill to the limit and learn to gauge approximately the strength of the double refraction shown. the author has found this to be a most valuable accomplishment. the more thing that can be learned with a pocket lens the better-even if one possessed other apparatus this will not usually be available in an auction room or on other people's premises. this same test can, of course, be extended and rendered easier b using a microscope, which functions as a much more powerful lens, magnifications of 25 to 60 diameters being quite comfortable usable with stone held between the thumb and forefinger of the left hand, supported on the microscope

Indian shadi Jewellery. 
By Aamir Mannan.
10.REFRACTIVE INDEX AND ITS MEASUREMENT

If therefor in any two media which are in optical contact we can measure the angle where total reflection beings (i.e. the critical angle), and we known the refractive index of the denser medium. it is possible to calculate the refractive index of the rarer medium. this is the underlying principle of all total 

reflection refractometers (sometimes called total refractometers), but in the instruments designed for gem testing all calculation is ingeniously avoided. In such refractometer the optically dense medium of known refractive index is in the form of polished hemisphere of heavy lead glass, or a segment of such a hemisphere, or a truncated 60o prism of the same material, the flat upper surface in each case forming the 'table' of the instrument. if the flat, polished surface of any gemstones of lower refractive index than the hemisphere is place in optical contact with this table, rays passing through the glass to the stone will be mostly

 Goa  Stylish Jewellery.
By Aamir Mannan.

Thus the ray AO is refracted along OA', apart from the small fraction of light which is reflected at O, as indicated by the dotted line. similarly, the ray BO is refracted along OB', as we consider rays which form greater angles with the normal, NOM, we reach an angle where the refracted ray only just grazes along the surface, OQ, between the two media. this angle is know as the critical angle, and all ray reaching O form the denser medium at angle greater than the critical angle are totally reflected back into the denser medium, it being physically 

impossible for any light form such rays to penetrate into the rarer medium. in the post, the angle COM can be considered as the critical angle of incidence; all the light s thus totally reflected along OC", and any other rays, such as DO, with a still larger of incidence, is, of course, also totally reflected. it is important to realise that the size of the critical angles will depend upon the

 Bihari Bridal Jewellery. 
By Aamir Mannan. 

Colours plays an extermely important part in contributing to the beauty and popularity of precious stones. it was their colour that made such stones as turquoise and lapis lazuli among the first to attract the cupidity of early civilised man, and it is the mangnificent crimson red of ruby, the deep cornflower blue of the finest sapphires and the verdant green of emerald which (added to their transparency, hardness and rarity) have caused them always to rank amongst the supremely precious gems. only in the case of diamond, unique in this as in so many other ways, has a complete lack of colour been  regarded as the standard of perfection. those unversed in gemmology find it hard to realise that is most of the mineral species used as gems a wide colour range is possible, and that in their 'pure' state they would be colourless and thereby of 

relatively little value.It has already been stated that each mineral has a definite refractive index by which it can be identified on the refractometer; list of these indices will be found on p. 27, and at the of the post. for the benefit of those who like to have at least a rough idea of the working of an instrument which they are using, a short description of the basic principle upon which all gem-tasting refractometer depend followers. however, those, who would like to know how to use a refractometer, but who prefer to shirk such explanation, can skip the next section and await the strictly practical directions which will be given later, on

Tripura Bridal Wedding Jewellery.

By Aamir Mannan.

Colours plays an extermely important part in contributing to the beauty and popularity of precious stones. it was their colour that made such stones as turquoise and lapis lazuli among the first to attract the cupidity of early civilised man, and it is the mangnificent crimson red of ruby, the deep cornflower blue of the finest sapphires and the verdant green of emerald which (added to their transparency, hardness and rarity) have caused them always to rank amongst the supremely precious gems. only in the case of diamond, unique in this as in so many other ways, has a 

complete lack of colour been  regarded as the standard of perfection. those unversed in gemmology find it hard to realise that is most of the mineral species used as gems a wide colour range is possible, and that in their 'pure' state they would be colourless and thereby of relatively little value.It has already been stated that each mineral has a definite refractive index by which it can be identified on the refractometer; list of these indices will be found on p. 27, and at the of the post. for the benefit of those who like to have at least a rough idea of the working of an instrument which they are using, a short description of the basic principle upon which

 West Bengal Bridal Jewellery.
By Aamir Mannan.

In some minerals the figures very somewhat; thus in zircon, while the blue, white, and golden 'fried' type most popular in moderan jewellery are almost constant in their birefringence of 0.059. many of the ceyclonese 

zircons. particularly the green varieties, have a much lower double refractions, sinking in some cases practically to zero. this peculiaity is unique amongst minerals, and will be discussed at greater length in the chapter on zircon. synthetic rutile has been manufactured in the USA (since 1949), and its anormous double refraction will be one means of identification. calcite, though not a gemstone, plays an important part in gemmological instruments in its optocally pure form of 'ice land spar', how, apart from the refractometer, can one test whether a given stone doubly refractive or not? the simplest method of all, which with a little

Indian Kerala Bridal Jewellery.

By Aamir Mannan.

This deviation of rays of light on entering a new medium is called refraction. In figure 2.1 the rays of light, I.O, is refracted along OR  on entering the denser medium blow the surface PQ. The broken line show the path of  followed by the reflacted light. The extent of the bending of refraction of light on entering the stone depends upon its refracting 

power or 'refractive index', and this is inversely proportional to the velocity of light withing the substance. Put differently, the refractive index of a medium may be define as the velocity of light in air* divided by the velocity of light in the medium. The velocity of light in air approximately I86,000 miles per second, and light from the sun and stars travel to us at this immense speed. In quartz (rock, crystal, amethyst, etc) the velocity is reduced to approximately I20,000 and its diamond to only 76,860 miles per second. Thus diamond, in which light travels as mentioned above, at 76,860 miles per second in empty space, has

Andhra Pradesh Bridal Jewellery.
By Aamir Mannan.
9.COLOUR, COLOUR FILTERS, AND THE DICHROSCOPE
It is the presence of traces of these in an otherwise colourless crystal that produces their colour varieties. more will be said about the function of these 'transition' element , when dealing with the spectroscope. granted that there are a considerable number of red, blue and green gemstones, there are very few that can approach ruby, sapphire and emerald in their rich shades of these colour and a colour sensitive and trained eye can go a long 


way towards distinguishing them from other natural gemstones, glass imitations, or even synthetic counterparts. it is indeed worthwhile for a beginning gemmologist to practice at every opportunity the sight identifications of the various gems and to pay special attention to the particular grades of colour typical of each species.

Gujarati Bridal Jewellery.
By Aamir Mannan.
8.DOUBLE REFRACTION AND DISPERSION

Thus is must be realised that if, say, a zircon is cut with is table facet at right-angles to optic axis it may be necessary to tile the stone and make observations trough the bezel facets to see the double refraction well developed. also, as R. T. liddicoat has pointed out, in directions at right-angle to the optic axis of the uniaxial minerals, although the double refraction is at is greatest is cannot be directly observed, as the image due to the extraordinary ray is either directly in front or directly behind 

that the due to the ordinary ray. with zircon, sphene, and peridot the birefringence  is so strong that even the novice should have no difficultly in seeing the effect mentioned effect in quartz and corundum unless the stone are larger. (J.-P. Poirot, director of the paris gem-testing laboratory, has made the important observation that  for a given birefringence the doubling effects is more marked in stones of low R.I. than in those of high index.)

 Uttar Pradesh Bridal Jewellery. 
By Aamir Mannan.
8.DOUBLE REFRACTION AND DISPERSION

Thus is must be realised that if, say, a zircon is cut with is table facet at right-angles to optic axis it may be necessary to tile the stone and make observations trough the bezel facets to see the double refraction well developed. also, as R. T. liddicoat has pointed out, in directions at right-angle to the optic axis of the uniaxial minerals, although the double refraction is at is greatest is cannot be directly observed, as the image due to 

the extraordinary ray is either directly in front or directly behind that the due to the ordinary ray. with zircon, sphene, and peridot the birefringence  is so strong that even the novice should have no difficultly in seeing the effect mentioned effect in quartz and corundum unless the stone are larger. (J.-P. Poirot, director of the paris gem-testing laboratory, has made the important observation that  for a given birefringence the doubling effects is more marked in stones of low R.I. than in those of high index.)

Maharashtrian Bridal Jewellery. 
By Aamir Mannan.
7.REFRACTIVE INDEX AND ITS MEASUREMENT

Thus the ray AO is refracted along OA', apart from the small fraction of light which is reflected at O, as indicated by the dotted line. similarly, the ray BO is refracted along OB', as we 

consider rays which form greater angles with the normal, NOM, we reach an angle where the refracted ray only just grazes along the surface, OQ, between the two media. this angle is know as the critical angle, and all ray reaching O form the denser medium at angle greater than the critical angle are totally reflected back into the denser medium, it being physically impossible for any light form such rays to

Indian Rakhi Gents Bracelet Jewellery.
6.COLOUR, COLOUR FILTERS, AND THE DICHROSCOPE
By Aamir Mannan.

While ruby and supphire are the red and blue varieties of the mineral corundum, there are also white, yellow, pink and green supphires. emerald and aquamarine are the rich green and pale blue varieties of beryls, but there are also white, pink, and yellow beryls, and green beryl that do not rank as emeralds, tourmaline, quartz, zircon and topaz are further well-known examples of such polychrome minerals, and tourmaline in particular 

is well known for frequently appearing as parti-coloured crystals, the boundaries between the colours -(usually green and pink) being quite sharp either across the length of the crystal or concentrically in zones. the reason of such confusing behaviour can usually be explained by the fact that there are a limited number (eight to be precise) of elements that give rise to colour in minerals.

Indian Assam Bridal Wedding Jewellery.
By Aamir Mannan.
4.REFRACTIVE INDEX AND ITS MEASUREMENT

It has already been stated that each mineral has a definite refractive index by which it can be identified on the refractometer; list of these indices will be found on p. 27, and at the of the post. for the benefit of those who like to have at least a rough idea of the working of an instrument which they are using, a short description of the basic principle upon which all gem-tasting refractometer depend followers. however, those, who would like to know how to use a refractometer, but who prefer to shirk such explanation, can skip the next section and await the strictly practical directions which will be given later, on