58 



MINERALOGY. 



pyramids, but modified so as to present much variety. 

 Some of the forms are represented in Fig. 22. Some of 

 the crystals have the pyramidal terminations at both 

 ends. 



Fig. 22. 



Nothing can be more pure and transparent than clear, 

 uncolored, limpid quartz. The common name for it is 

 rock crystal. It is said that it was to this mineral that 

 the ancients first gave the appellation of crystal, from its 

 resemblance to perfectly clear ice, the Greek word for 

 ice being Jcrustattos. It is often used in jewelry, and also 

 to make lenses for optical instruments. The amethyst, 

 so called, is a variety having a purple or red color from 

 the presence of some oxyd or oxyds. The rich purple 

 specimens, so much prized for jewelry, derive their color 

 from the oxyd of manganese. Sometimes the crystals 

 of quartz have a light yellow color, and then are called 

 false topaz. Smoky quartz appears with various degrees 

 of color. Crystals with the lighter shades are often very 

 beautiful, and are sometimes used in jewelry. Sometimes 

 quartz is filled with golden-yellow spangles of mica; but 

 the artificial imitations of this mineral, contrary to the 

 general fact that nature excels art, transcend in beauty 

 the original. The colors of what is called ferruginous 

 quartz, yellow, brownish-yellow, and red, are produced 

 from oxyd of iron, and hence its name,/emm being the 

 Latin word for iron. 



114. Chalcedony. This kind of quartz, as already 

 stated, does not appear in crystalline form, but in spher- 



