158 ALUMINA. 



mineral has been called Brucite ; but chondrodite is of prior 

 authority ; it is from the Greek chondros, a grain. 



Obs. It is abundant in the adjoining counties, Sussex, N. 

 J., and Orange, N. Y., occurring at Sparta, and Bryam, N. 

 J., and in Warwick and other places in New York. At 

 Vesuvius it occurs in small crystals, called Humite. 

 4. ALUMINA. 



1. Uncombined. 



CORUNDUM. 



Rhombohedral. R : R=86' 4. Cleavage sometimes 



„ ^ perfect parallel with a. Usual in six-sided 



prisms, often with uneven surfaces, and 

 sometimes so irregular that the form is 

 scarcely traceable. Occurs also granular. 

 Colors blue, and grayish-blue most com- 

 mon ; also red, yellow, brown, and nearly ^s=h^ 

 black ; often bright. When polished on the surface a, a star 

 of six rays, corresponding with the six-sided form of the 

 prism, is sometimes seen within the crystal. Transparent 

 to translucent. H = 9, or next to the diamond. Exceed- 

 ingly tough, when compact. Gr=3*9 — 4*16. 



Composition : pure alumina. It remains unaltered before 

 the blowpipe both alone and with soda. Fuses with diffi- 

 culty with borax. 



Varieties. The name sapphire is usually restricted in 

 common language to clear crystals of bright colors, used as 

 gems ; while dull, dingy-colored crystals and masses are 

 called corundum, and the granular variety of bluish-gray and 

 blackish colors is called emery. 



Blue is the true sapphire color. When of other bright 

 tints, it receives other names ; as oriental ruby, when red ; 

 oriental topaz, when yellow ; oriental emerald, when green ; 

 oriental amethyst, when violet ; and adamantine spar, when 

 hair-brown. Crystals with a radiate chatoyant interior are 

 often very beautiful, and are called asieria, or asteriated 

 sapphire. 



What is the usual form of crystals of corundum ? What are their 

 oiors I hardness ? Of what does sapphire consist ? What are the red, 

 ellow and green varieties called? What the hair-brown variety! 

 «Vhat are conmdum and emery ? What is asteriated sapphire? 



