(RUBY, SAPPHIRE, ETC.)
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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