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THE EMERALD 95
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glass.
The hardness varies from 7-1/2 to 8, the crystals being very brittle,
breaking with a fracture of great unevenness. The better varieties are
transparent, varying from that to translucent, and are called the "
noble " beryls. Transparent beryl crystals are used by fortunetellers
as " gazing stones," in which they claim to see visions of future
events.
The
Emerald. Considering the particular emerald which is a variety of
beryl—although the name emerald in the trade is applied somewhat
loosely to any stone which is of the same colour, or approaching the
colour of the beryl vavietv—this emerald only differs chemically from
the beryl, just described, in possessing an addition of oxide of
chromium. In shape, crystallisation, fracture and hardness, it is the
same, and often contains, in addition to the chromium, the further
addition of traces of carbonate of lime, magnesia, and occasionally
faint traces of hornblende and mica, which evidently result from its
intimate association with the granite rock and gneiss, amongst which
it is mostly found, the latter rocks being of a slaty nature, in layers
or plates, and, like granite, containing mica, pyrites, felspar,
quartz, etc.
Emeralds
have been known horn very early times, and are supposed to have been
found first in the mines' of ancient Egypt. They were considered
amongst the rarest and the most costly of gems, and it was the custom,
when conferring lavish honour, to engrave or model emeralds for
presentation purposes. Thus we find Pliny describes Ptolemy giving
Lucullus, on his landing
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