Indian Bihari Bridal Jewellery Collection 33

 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

p.21. what followers can best be understood by considering the diagram, figure 2.2. here rays of light are

considered passing from a dense medium into a rarer one, say from glass into air. the rays will then be refracted away from the normal, NOM (the revers process of the former case considered in figure 2.1). As would appear in a singly refractive stone such as diamond, each edge appears as a double line (see figure 3.1). a word of warning, however, should be uttered here; according to their crystal structure, all doubly refracting stones have either one or two direction in which only single refraction obtains. those gemstones belonging to the tetragonal, trigonal or hexagonal system of crystals have only such direction; one 'optic axis' as it called. these minerals, which are grouped together optically us 'uniaxal', include zircon, corundum (ruby and sapphire), beryl (emerald and aquamarine), tourmaline, and quartz (amethyst and citrine). the minerals crystallising in either the orthorhombic, monoclinic or triclinic systems have two optic axes, and are called 'biaxial'. these include peridot, chrysoberal (alexandrite), topaz, and spodumene (kunzite) among the coommoner gemstones.