QUARTZ 473 



cases the coloured varieties also freely allow the passage of light. These transparent 

 varieties of quartz are distinguished as precious quartz; between precious quartz and 

 cloudy or opaque common quartz every degree of transparency is to be found. 



Chemical examination of the coloured varieties of quartz shows that their colour is 

 due to the presence of impurities intermixed with the colourless cjuartz substance. In 

 some cases the pigment, the nature of which is unknown, is in the finest state of sub- 

 division, and is distributed uniformly throughout the mass of the stone, so that it is 

 impossible to distinguish individual particles even with the aid of the highest powers of 

 the microscope. In other cases the colouring matter is seen under the microscope to be 

 located in needles, fibres, grains, scales, &c., of other mineral substances, which are 

 embedded in the quartz substance in such numbers as to impart to it their own colour. To 

 the former class belong brown smoky-quartz, violet amethyst, yellow citrine, and rose-red 

 rose-quartz, and to the latter gi-een prase, blue sapphire-quartz, and others. The range of 

 colour shown by quartz is very extensive ; no colour is entirely unrepresented, and the 

 majority exist in a number of diff"erent tints and shades. There is very often a marked 

 contrast between the colour, or the shade of colour, of different portions of the same 

 specimen ; this is shown most typically in agate, that variety of quartz so much used for 

 ornamental piu-poses. The most important of the coloured varieties of quartz have been 

 enumerated above. 



Optically, quartz, being a hexagonnl mineral, is doubly "^-acting, but neither the 

 refraction nor the double refraction is very strong. An inspection of the table given 

 below will show how small is the difference between the ordinary and the extraordinary 

 refractive indices for light of different colours : 



Red light . 

 Yellow light 

 Green light 

 Blue light . 

 Violet light 



Although the double refraction of quartz is not very great, it is strong enough to make 

 the images of any small object, such as a candle-flame, appear double when viewed through 

 a transparent faceted stone (Fig. 26«). By this means quartz may be distinguished from 

 glass of similar colour, since this will show only single images (Fig. 266). A comparison 

 of the numbers in the table given above shows further that there is little difference 

 between the refractive indices of quartz for light of different colours, and consequently that 

 the dispersion of this mineral is feeble, so that it exhibits no play of prismatic colours 

 comparable to that of the diamond. 



A peculiar optical feature possessed by quartz, and by only one other mineral, is the 

 power of rotating the plane of polarisation of polarised light, but this being of purely 

 physical interest need not be dwelt upon here. 



The characters common to all varieties of quartz having been described, we must now 

 direct our attention to the features which distinguish the ornamental from each other and 

 from the common varieties. Of the ornamental varieties of quartz there are first those 

 which occur in regular crystals, or in aggregates of crystals, the individuals of which are 

 recognisable as such by the naked eye, though they may sometimes have irregular 

 boundaries. Next we have the compact varieties, consisting of an aggregate of numbers 

 of quartz crystals of microscopic size. The crystallised varieties may be further sub- 

 divided according to colour, and the compact aggregates according to structure and other 



