32 



BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Crystallization. — Hexagonal-trigonal, rhombohedral ; habit usually 

 pyramidal, with rounded edges, and with prominent basal plane- 

 (See fig. 4.) 



Color. — Colorless when pure; often showing more or less intense 

 coloration, and named accordingly; ruby, the red variety colored 

 by chromium; sapphire, blue, titanium; oriental amethyst, violet, 

 chromium and titanium; oriental emerald, green, iron and titanium; 

 and oriental topaz, yellow, iron. May also show silvery internal 

 reflections, usually in the form of a six-rayed star, owing to symmet- 



Fig. 4.— Corundum crystals. 



rically arranged inclusions (asteria); pleochroic from pale to deep 

 tints. 



Luster. — Rather dull and greasy in natural state; adamantine 

 when cut; transparent to translucent. 



Hardness. — 9; will scratch every mineral except diamond; ex- 

 tremely durable. 



Specific gravity. — 4 ±0.1; noticeably heavy. 



Optical properties. — Mean refractive index, 1.76; double refraction 

 weak, 0.008; uniaxial, negative. 



The transparent corundums rank among the most valuable of gem 

 stones, and include two recognized varieties, the red ruby and the blue 

 sapphire. The ruby varies in hue from a rose to a deep carmine, the 



