ALUMINA. 



315 



Varieties. Sapphire. This name is applied to the transparent blue varieties. 



Oriental Ruby, is transparent, and of a blood-red or rose-red colour. 



Emery. Massive ; nearly opaque ; bluish grey and greyish black. 



Common Corundum. Massive and crystallized ; of various colours ; translucent. 



Adamantine Spar. This is brown and faintly translucent. 



Composition. Sapphire — Alumina 98.50, oxide of iron 1.00, lime 0.50 [Klaproth). 



Emery — Alumina 86.00, silica 3.00, lime 3.00, oxide of iron 4.00 {Tennant). In its 

 pure form, therefore, this mineral is an oxide of aluminum, the formula of which, according 

 to Kane, is Al O3. 



Geological Situation. The finest sapphires and rubies occur in alluvial soil in Ceylon, 

 Sec. Common corundum and adamantine spar are said to be found at foreign localities, in 

 granite. In this State, and in New- Jersey, corundum has heretofore been found only in white 

 limestone. 



Uses. The transparent varieties are highly esteemed as gems. The common corundum, 

 and emer}% are employed for polishing steel, cutting gems, etc. 



Fig. 254. 



localities. 



Orange County. Corundum has been found, by Dr. 

 Horton, in the vicinity of the village of Amity. The 

 colours are nearly white, pale and deep blue, and pale 

 and deep rose-red. It occurs in crystals and grains. Fig. 

 253 and 254 represent two of the forms which have been 

 observed, but the sides are rough, and cannot be well 

 measured. It is associated with hornblende, spinelle and 

 red oxide of titanium, in white limestone. This mineral 

 is also found at Newton in New-Jersey. 



Fig. 253. 



<: 



SPINELLE. 



Spinelle. CUaveh/nd and Beudant. — Spinell. Thomson. — Spinel. Phillips, Shepard and Dana.  

 hedral Corundum, or Spinel. Jameson. — Dodecaedrischer Korund. Mohs. 



Fig. 255. 



 Dodeca- 



Description. Colour red, sometimes green, black, blue, yellow, 

 brown, and nearly white. Streak white. It occurs regularly crys- 

 tallized. The primary form is a regular octahedron. Fig. 255. 

 P on P' 109° 28' \&' {Hauy). Cleavage parallel with the primary 

 from difficult. Fracture conchoidal. Lustre splendent and vitreous. 

 Form transparent to translucent on the edges. Hardness 8.0. 

 Specific gravity from 3.50 to 3.60. Infusible by the blowpipe 

 alone ; with borax, it fuses slowly into a transparent glass. The 

 solution obtained by an acid after treatment with caustic potash, 

 affords a precipitate of alumina after the addition of hydrosulphate of ammonia. 



