MINERALOGY. 429 



SECOND ORDER. 

 ALUMINA (CRYSTALLIZED). 



I. Precious minerals belonging the the Clay Alu- 

 mina. 



Corundum of which there are several varieties, is, 

 except the diamond, the hardest substance known ; 

 H. = 9.0, G. 3.9 to 4.0. Found in large crystals ; 

 luster translucent, compact, presents internal reflections 

 only in certain portions. Corundum or the Sapphire 

 species is pure alumina crystallized ; sometimes found 

 bright-colored, mostly grayish-brown, seldom colorless. 

 Fracture conchoidal, imperfect in the blue variety ; spe- 

 cies easily recognized by their several characteristics. 

 The dingy, blue-gray massive variety of coarse granular 

 structure, called Emery, is found in large boulders in 

 Saxony and the island of Naxos. From its extreme 

 hardness its powder is largely employed in the polishing 

 of glass and metals, and in the cutting of gems and 

 other minerals. 



Demantspath Adamantine Spar, a rough, coarse, 

 crystalline stone of dingy hue, easily divisible from the 

 other varieties, is employed in cutting and polishing dia- 

 monds. The name Sapphire is sometimes used for the 

 whole coruncl genus, but the true Sapphire is of a beau- 

 tiful blue color and remarkable transparency. The yel- 

 low variety is known as Oriental Topaz ; the red is called 

 Ruby, and nearly rivals the diamond in value. The 

 more common kinds of sapphire and ruby are employed 

 in the manufacture of watches. These gems, although 

 beautiful, are less valued as toilet ornaments than dia- 



