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182
GEMS.
gave
it we cannot know where they procured it, hut fine pieces, hoth rough
and worked, are often found amongst the remains of Italian monuments.
It
appears that the plasma was only used ornamentally during the time of
the Lower Roman Empire. as the subjects of the ancient intagli and the
quality of the execution bear witness. Contrary to my opinion, to which
for the above mentioned reasons I firmly adhere, Emmanuel asserts that
even the Greeks called this stone plasma, because its name was derived from the word πλάσμα, image; and
Barbot, misled by the same ambiguity, says that the plasma comes only
from India, and that in ancient times it was probably thence brought to
wealthy Carthage, and from there to Rome.
LXXXII.
PORPHYRY.
Although porphyry
is not a gem, we think it right to make some mention of it, as we did
of marble, as much for the purpose of following the example of those
who have written on the subject of gems as because many works of art
have been executed in that material.
Porphyry is composed of a felspar rock called, scientifically, leptinite, mixed
with crystals of ortosa and albite. The different kinds are
distinguished by their ground colour, in red, green, white, brown, or
black.
Their specific gravity varies, according to their aggregated substance, from 2-69 to 2-77.
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