342 MINERALOGY 



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and colored brown or flesh-colored from the iron content, Fe 2 03 

 being isomorphous with A1 2 3 . Clear corundum is variable in 

 color and runs through the entire list of colored precious stones. 

 When it is wished to convey the idea that the gem in question is 

 corundum, the term "oriental" is prefixed, as "oriental topaz," 

 " oriental amethyst," or " oriental emerald," etc. The true ruby is 

 corundum; and when of that dark " pigeon blood " red, so much 

 sought after and of the right transparency, it is the most expensive 

 of gems, even surpassing the diamond of the first water in value. 



The ruby is colored with small quantities of chromium, while the 

 sapphire and other colors are due to cobalt, titanium, or iron. 



Its physical properties, together with its wide range of colors, 

 render the transparent varieties of corundum an ideal gem stone. 

 It is the third hardest substance known, being surpassed only by 

 the diamond and silicon carbide, a product of the electric furnace. 



Corundum is a primary mineral of such igneous rocks as granites, 

 syenites, rhyolites, and rocks rich in alumina. 



In thin sections it appears transparent and nearly colorless, with 

 or without crystalline outline, and there are no characteristic in- 

 clusions. Relief is strong, and the interference colors are of the 

 first order yellow or gray. 



In addition to being a primary mineral of igneous rocks, it is also 

 a characteristic mineral of the belt of metamorphism, and is there 

 associated with tourmaline, spinels, cyanites, garnets ; while in its 

 decomposition by weathering it forms a whole series of aluminous 

 minerals, as gibbsite, diaspore, margarite, muscovite, etc. 



Occurrence. Most of the gem material is of Eastern origin; 

 the best rubies are found in the gravels of Irrawaddy River, near 

 Mandalay, Burma, and in the crystallized limestone on its eastern 

 bank ; the crystals are tabular in habit and associated with spinels 

 and garnets. Those recovered from the river gravels are rounded 

 and water-worn, but owing to their excessive hardness some still 

 retain their crystalline faces. In Ceylon the gemstones are also re- 

 covered mostly from the gravels of the Ratnapura and Rakwena 

 districts. In the United States sapphires are found in the gravel 

 bars of the upper Missouri River in Montana, and stones of gem 

 value are mined in the Judith River valley, Montana. These are 

 contained in a dyke cutting through a crystalline limestone. The 

 dyke having weathered faster than the limestone may be traced by 

 a depression across the country for five miles, and many sapphires 

 have been taken from the piles of dirt at the entrance to the gopher 



