54 OUR PHYSICAL WORLD 



layer of quartz, the innermost layer often being a layer of 

 upstanding crystals. Later on, the rock containing such a 

 hollow mass of quartz may disintegrate, freeing the quartz mass. 

 Such a rounded chunk with a hollow center lined with crystals is 

 known as a geode. 



The layers of quartz deposited in a rock cavity or at the sur- 

 face about a hot spring may have a waxy luster. Such quartz 

 is known as chalcedony. 



But the quartz in the process of solution in water and rede- 

 posit is very prone to become impregnated with impurities that 

 color it. So quartz either in the massive or crystalline condition 

 may assume almost any color. A very beautiful variety of mas- 

 sive quartz is tinged with pink and is known as rose quartz. 

 Quartz crystals may be tinged with purple and are then called 

 amethysts. They are so beautiful as to be in demand for gems. 

 So the crystals when tinged with yellow are mounted as topaz, 

 although they are false topaz, as the real gem is still harder and 

 more lustrous than quartz. Similarly, red quartz crystals 

 make false rubies: green, false sapphires. The crystals may be 

 hazy with dark coloring and are then known as smoky quartz. 



The layers of quartz deposited in cavities or about the mouths 

 of hot springs may be colored with different tints as first one 

 impurity, then another, is predominant. If the layers are vary- 

 ing shades of red, onyx is produced. Rounded masses of quartz 

 deposited in varicolored layers in some small .cavity of the rock 

 and later set free by rock disintegration are known as agates. 

 The layers may be shades of red, varying degrees of dark colors, 

 blues, or yellows. Such an agate may look like an ordinary 

 rounded quartz pebble or bowlder when found, for the exterior 

 is rough and water-worn, but when broken open it displays the 

 concentric colored layers. When ground down and well polished 

 it is a thing of marvelous beauty. Some very exquisite vases and 

 bowls are made of agate, chalcedony, and onyx, and the latter is 

 used for table tops or even decorative pillars in the interiors of 

 costly buildings. 



