180 NATURE STUDY 



a six-sided pyramid ; but this typical form is rarely seen. 

 Often there are only the pyramids. This is called drusy 

 quartz. The form most commonly seen is the prism 

 terminated with a pyramid. Quartz crystals, when col- 

 ored violet or purple, are known as amethysts. 



Other well-known forms of quartz are rose quartz, 

 smoky quartz, yellow quartz, agate, chalcedony, carnelian, 

 opal, onyx, flint, jaspar. The stiffness of grass blades 

 and the grain is due to silica or quartz. 



Some of the uses of quartz have already been given 

 under the head of Sand. Eock crystal was once used 

 extensively for the best spectacles. What the boys call 

 " reallys " (marbles) are made from agate, which is used, 

 too, for various ornamental purposes, as are also car- 

 nelian, amethyst, jaspar, chalcedony, onyx, and opal. 

 From flint were made the Indian arrow heads. The white 

 men used it for striking a fire. 



Feldspar : 



The commonest feldspar is that known to mineralo- 

 gists as orthoclase. It is softer than quartz, harder 

 than glass, ranging from six to seven in a scale of hard- 

 ness, which is as follows : 



1. Chalk, Talc, Graphite. 



■ 2. White Mica, Rock Salt, 



3. Calcite. 



4. Fluorite. 



5. Glass. 



6. Feldspar (usually). 



7. Quartz. 



8. Beryl. 



9. Corundum. 

 10. Diamond. 



