CORUNDUM AND SPINEL. 



1 



CORUNDUM. 



The mineral species Cornndum affords 

 a number of gems known by different 

 names. These differences arise from the 

 fact that the stones were used as gems 

 before their mineralogical identity was 

 discovered. Thus red Corundum is 

 known as the ruby and blue Corundum 

 as the sapphire. When Corundum suit- 

 able for gem purposes occurs of other 

 colors, such as green, yellow or violet, 

 the gems are sometimes known as green, 

 yellow or violet sapphires, respectively, 

 or b}^ the name of another gem which 

 they closely resemble in color, with the 

 adjective Oriental prefixed. Such are 

 the gems known as Oriental topaz, Ori- 

 ental emerald. Oriental aquamarine, Ori- 

 ental hyacinth. Oriental amethyst and 

 Oriental chrysolite. Colorless Corun- 

 dum is known as leucosapphire. 

 While Corundum of all colors is used for 

 gems, it is only that which is transpa- 

 rent which can be so employed. This is 

 sometimes called noble Corundum to dis- 

 tinguish it from common Corundum. 

 The two, however, often occur together. 

 Common Corundum ^is used as an abra- 

 sive, emery being one of its varieties, 

 but it has no gem value. 



Corundum is a sesquioxide of alumi- 

 num, with the percentages aluminum 

 53.2, oxygen 46.8. Its hardness is 9 

 in the scale of which diamond is 10, and 

 no other mineral except the latter equals 

 it in hardness. This hardness gives it 

 a wearing quality as a gem second only 

 to the diamond. The varieties of Corun- 

 dum differ slightly in hardness, sapphire 

 being the hardest. Noble Corundum has 

 a brilliant, vitreous luster, which, while 

 not equal to that of the diamond, is supe- 

 rior to that exhibited by almost any other 

 gem. Corundum is a heavy mineral, its 

 specific gravity being four times that of 

 water. This high specific gravity affords 

 an easy means of distinguishing the gems 

 of Corundum from those of other spe- 

 cies. Corundum is infusible and is not 

 attacked by acids. It crystaUizes in 

 the rhombohedral division of the hex- 

 agonal system, certain crystal forms 

 being characteristic of the two varieties. 



ruby and sapphire. Thus ruby tends to 

 crystallize in flat rhombohedral crystals, 

 while sapphire generally forms in long, 

 while sapphire generally forms in 

 longer, hexagonal prisms. (See col- 

 ored plate in Novem.ber number.) 

 Corundum is doubly refracting and 

 dichroic. Of the different colors 

 of Corundum above referred to, the blue 

 or sapphire is most common, the red or 

 ruby next. The other colors occur 

 rather sparingly, green having been al- 

 most unknown until the discovery of the 

 Montana sapphires. The nature of the 

 coloring ingredient of the different varie- 

 ties of Corundum is not known, but there 

 is some reason for believing it to be chro- 

 mium, for Fremy obtained artificial red 

 and blue Corundum by mixing chromium 

 with his other ingredients, after many 

 attempts to obtain the desired color had 

 failed. 



Red Corundum varies in hue from 

 rose to deep red. That of the lat- 

 ter tint is the true ruby, the color known 

 as pigeon's blood being most highly 

 prized. Faultless stones of this color have 

 long been the most valuable of gems, ex- 

 ceeding the diamond in price, weight for 

 weight, unless the latter is colored. 

 Rubies above three carats in weight are" 

 about ten times more valuable than ordi- 

 nary diamonds of the corresponding 

 weights. But few rubies excseding ten 

 carats are known. The King of Pegu is 

 reported to have one the size of a hen's 

 egg, but as no one has ever seen it the 

 story may well be doubted. In the crown 

 of the Empress Catherine was, however, 

 one the size of a pigeon's egg. There is 

 also a large uncut ruby in the British 

 crown, which Ruskin calls the loveliest 

 precious stone of which he has any 

 knowledge. 



The chief home of the ruby is Burmah. 

 From its mines and those of Siam and 

 Ceylon have come practically all the 

 world's supply of rubies. The most im- 

 portant Burmese mines are in Mogouk, 

 ninety miles north of Mandalay. The ru- 

 bies were evidenth^ formed in limestone, 

 which is now much decomposed, and 

 seem to have been the result of metamor- 



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