QUARTZ 



471 



QUARTZ. 



No other mineral exists in such abundance, and at the same time affords so many 

 different ornamental stones as does quartz in its several varieties. From one point of view 

 quartz is one of the most important of precious stones, not on account of its value but 

 because of its very wide distribution, and also because of the occurrence of even the most 

 beautiful varieties in large masses. For these and other reasons all the ornamental varieties 

 of quartz are classed with the so-called semi-precious stones. 



In order to avoid repetition the characters common to all the varieties of this mineral 

 Avill be first described, and afterwards the features which distinguish the varieties used as 

 gems and for ornamental purposes. 



Quartz is pure silica, that is to say, it is oxide of silicon with the chemical formula SiOj, 

 and consists, in the purest condition, of 46*7 percent, of silicon and 53-3 per cent, of oxygen. 



Fig. 8i5. Crystalline forms of quartz. 



More frequently than not it contains impurities of various kinds, a fact to which the variety 

 in colour of the mineral is due. Quartz differs from opal, another mineral which consists 

 lai'gely of silica, in that it is completely free from water. 



Quartz differs also from opal in another impoi'tant respect, namely, in that it is not 

 amorphous but crystallised. Very finely developed crystals of quartz, usually with brilliant 

 faces, are extremely common. They belong to the hexagonal system (Figs. 85a — J), and 

 almost without exception take the form of regular six-sided prisms, the faces of which are 

 characterised by the presence of very distinct striations perpendicular to the edges of the 

 prism. The latter is terminated at one end (Figs. 85& - d), and in completely developed 

 crystals (Fig. 85a) at both ends, by a six-sided pyramid, the faces of which intersect the 

 prism-faces in horizontal edges. In addition to these common faces there are often others of 

 small size (Figs. 536 - d), the arrangement of which indicates that the symmetry of quartz 

 is that of the ti'apezohedral-tetartohedral division of the hexagonal system. The liabit of 

 quartz crystals is rather variable. In many cases the prism-faces are elongated, as in the 

 forms represented in the figures ; but sometimes they are short or even completely absent, 

 and in the latter case the crystal becomes a double six-sided pyramid. The faces of this 

 pyramid may be all of the same size, but more frequently large faces alternate with small 

 ones, so that there are three large and three small faces (Figs. 85b — d). Owing to irregular 



