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20
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GEMS.
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II.
AQUAMARINA.
This stone is so named from its colour, which so much resembles sea-water.
Like
other gems, it is divided into Oriental and Western. Amongst those
which, on account of their hardness, are called " Oriental," the most
hard is merely a light blue corundum with a slight tint of green and
yellow. It is easily known by its specific gravity, which is always
above 4. This gem is very scarce, and it would have all the value of
other corundums but for the colour which is common to a great number of
other stones possessing little value. The others are found in the
Island of Ceylon, and from it they take their name. They are of
a deeper greenish-blue colour, which renders them somewhat different
from the Western stones. Their specific weight varies from 3549 to
3-908, and it is thought that their hardness nearly equals that of the
Brazilian chrysolite. They resist the wheel more than the others, and
their brightness exceeds that of the Western gems. These are very
transparent, and take a beautiful polish notwithstanding their
inferior hardness, which is less than that of the topaz. The specific
weight is from 2-70 to 2-77. They possess double refraction,
but in a weak degree. The cleavage is brilliant and wavy, sometimes
scaly. Their primitive form is an elongated hexagonal prism. They melt
when exposed to fire and
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