Indian Hyderabad Bridal Jewellery Collection 18

Indian Hyderabad Bridal Jewellery.
By Aamir Mannan. 

REFRACTIVE INDEX AND ITS MEASUREMENT

The first task identifying a gemstones is to make quite sure to what mineral species it belongs: to determine, for instance, whether it is a corundum (ruby or supphire), a beryl (emerald or aquamarine), quartz (amethyst or citrine), topaz, tourmaline, spinal, peridot, or zircon.Though one may often make a shrewd guess as to the nature of a stone by reason of its colour, lustre, and general appearance, it is often only by measuring one or other of its optical or physicals 'constants' that one can be really sure of once's ground.In this chapter methods are described for measuring the most important of these constant.Men is a profoundly lazy creature and its has been said that most of his inventions has been born of the urge to save himself trouble.Presumably, therefor, jewellers who are not scientifically minded would welcome an apparatus which would enable stones to be identified in some really simple manners, quickly, clearly, and without calculations.If, merely by placing the 

table facet of an unknown stones on an instrument, its name could be read off by mean of a pointer on a calibrated scale, one would expect such an apparatus to be widely popular.Instrument closely corresponding to this 'ideal' have in fact been designed and marketed (at rather a high price) during recent years.These reflectivity meters aim to identify gemstones, where these have a clean and well polished table facet, by measured their reflecting power when a narrow bean of infra-red light impinges on this facet.Such instruments have certain distinct advantages over the traditions types of jeweller's refractometer that have been the mainstay

of gems identifications for three-quarters of a century.But they also have grave limitations, which are explained later in this chapter.The refractometer in still an essential instrument for the gemmologist, and the principles of its constructions and hoe to make the best use.of it are fully explained later.First, however we must explain the meaning of refraction and refractive index and the basic principles on which the refractometer operates.What happens when a ray of light fall on the surface of a transparent solid such as a gemstones or a sheet of glass? Some of the light is reflected at the surface of  the stone.The reflected  light leaving the surface at an angle equal to that at which it falls upon the surface ('incident angle' or 'angle of incident').It is the reflected light which provides the surface lustre of the stone.The greater proportions of the light, however, passes into the stone, but in this dencer medium it travelsmuch more slowly than in air.The effect of entering the dencer medium, in which their velocity is diminished, upon trains of light waves striking the surface obliquely is to alter their direction and make them follow a new path nearer to the perpendicular (or 'normal' as it is called) to the interface between the two media.