Indian kathak Fashion Jewellery.
By Aamir mannan.
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
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 refracted into the stone, and thance escape into the air, when they strike the surface at less than the critical angle, but totally reflected back from the surface of the stone when they strike at an angle exceeding the critical angle.
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 
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