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CORUNDUM
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The mineral species corundum affords a number of gems known by different names because the stones were used as gems before their miner-alogical identity was discovered. Thus the ruby is red corundum and sapphire blue corundum. When corundum suitable 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 by the name of another gem which they closely resemble in color, with the adjective "Oriental" prefixed. Such are the gems known as Oriental topaz, Oriental emerald, Oriental aquamarine, Oriental hyacinth, Oriental amethyst, and Oriental chrysolite. Colorless corundum is known as leucosapphire. While corundum of all colors is used for gems, it is only that which is transparent which can be so employed. This is sometimes called noble corundum to distinguish it from common corundum. The two, however, often occur together. Common corundum is used as an abrasive, emery being one of its varieties, but it has no gem value.
Corundum is a sesquioxide of aluminum, with the percentages, alumina 53.2, oxygen 46.8. Its hardness is 9 in the scale, and no mineral except the diamond equals it in this respect. This hardness gives it a wearing quality as a gem second only to the diamond. The varieties of corundum 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 superior 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 gems of corundum from those of other species. Corundum is infusible, and is not attacked by acids. It crystallizes in the rhombohedral division of the hexagonal 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 longer hexagonal prisms. (See colored plate.) 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
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