QUARTZ. 135 



imitations of this stone are more beautifm than the natural 

 aventurine. 



Ferruginous Quartz. Includes opaque, yellow, brcwnish- 

 yellow, and red crystals. The color is due to oxyd of iron. 

 These crystals are usually very regular in their forms, (fig 

 ure 2,) and not distorted like the limpid crystals. They are 

 sometimes minute and aggregated like the grains of sand in a 

 sandstone. 



Chalcedonic Varieties. 



Chalcedony. A translucent massive variety, with a glis- 

 tening and somewhat waxy luster ; usually of a pale grayish, 

 bluish, or light brownish shade. It often occurs lining or 

 filling cavities in amygdaloid and other rocks. 



These cavities are nothing but little caverns, into which 

 siliciceous waters have filtrated at some period. The stalac- 

 tites are " icicles" of chalcedony, hung from the roof of the 

 cavity. Some of these chalcedony grottos are several feet 

 in diameter. 



Chrysoprase. An apple-green chalcedony. It is colored 

 by nickel. 



Carnelian, A bright red chalcedony, generally of a clear 

 rich tint. It is cut and polished and much used in the more 

 common jewelry. The colors are deepened by exposure of 

 several weeks to the sun's rays. It is often cut for seals and 

 beads. The Japanese cut great numbers into beads of the 

 form of the fruit of the olive. 



Sard. A deep-brownish red chalcedony, of a blood-red 

 color by transmitted light. 



Agate. A variegated chalcedony. The colors are dis- 

 tributed in clouds, spots, or concentric lines. These lines 

 take straight, circular, or zigzag forms ;' and when the latter, 

 it is called fortification agate, so named fromjthe resemblance 



the angular outlines of a fortification. ». iThese lines are 

 the edges of layers of chalcedony, and these layers are the 

 successive deposits during the process of its formation. 

 Mocha stone or Moss agate is a brownish agate, consisting 

 of chalcedony with dendritic or moss-like delineations, of an 

 opaque yellowish brown color. They arise from dissem- 

 inated oxyd of iron ; all the varieties of agate are beau- 



"W hat is ferruginous quartz? Describe chalcedony. What is said 

 jf its formation ? What is chrysoprase ? What is carnelian '? How ia 

 its color deepened ] For what is it used 1 What is sard ? Describe 

 agate. 



