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126
GEMS.
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It
is formed in dodecahedral prisms with bright faces, and also in grains
and pebbles. Its breakage is concave ; it is transparent and
translucent ; of a strong vitreous lustre, and is composed of folds,
irregularly placed, one over another. In colour, it is sometimes blue ;
at other times white, green, grey, or black.
It
marks white glass deeply, but is sometimes marked by quartz, although
but slightly. Its dust is whitish. Friction confers on it negative
electricity.
Under
the action of the blow-pipe, it loses colour ; it does not yield water
when calcined, and it melts into a white porous glass. "When mixed with
borax and subjected to fire, it effervesces and forms a clear glass,
which becomes yellow when cool. With salt of phosÂphorus, it melts,
with effervescence, and forms an iridescent glass. Under the action of
acids, it is transÂformed into a gelatinous substance.
Its
specific weight varies from 2'60 to 3"33. It is found in liquefied
basalts, and in the vulcanized matter of Vesuvius, also at Bodemnaise,
at the Lake of Laach, in various parts of Italy, in the island of
Tiree, and in Scotland.
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