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204
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GEMS.
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Between
the emerald corundum and the Western emerald of fine quality there is
the same difference as between the perfect red corundum and the perfect
spinel.
The Western emerald is a silicate of alumina, almost identical in its composition with the beryl or aquamarine.
It is the least hard amongst gems, and therefore breaks easily.
It
crystallizes in truncated hexagonal prisms at the two extremities ; its
crystals lie between the composite rocks and argillaceous schists, and
also in the accidental cavities which occur in granite blocks.
Sometimes
it is found united with crystals of quartz, mica, and felspar. It is,
moreover, procured from sulphuretted iron, in carbonate of lime, and
in sulphuretted lime.
This emerald has a vitreous and seldom a resinous light. It is soft when just taken from the mine, but hardens in the open air.
Its specific gravity is between 2-73 and 2-76. Its hardness between 7-05
and 8. It yields a white powder. It becomes electric by fiiction. It
melts under the action of the blow-pipe, forming a limpid vesiculated
glass of the consistency of borax. When calcined and still hot, if
thrown into water, it breaks into fragments of various colours.
It gives, on analysis,
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