Alexandria, an emerald on which was engraved his portrait. Pliny also
relates how the short-sighted Nero watched the fights of gladiators
through an eye-glass made of an emerald, and in ancient times, in Rome,
Greece, and Egypt, eye-glasses made of emeralds were much valued. Many
of these, as well as engraved and carved emeralds, have been discovered
in ruins and tombs of those periods.
The copper emerald is rare : it is a hydrous form of copper silicate, CuOSiO2 + H2O,
of a beautiful emerald green, varying from transparent to translucent.
It exhibits double refraction, and is a crystallised mineral, brittle,
and showing a green streak. Tin's is less hard than the real emerald,
is heavier, deeper in colour, and is usually found in crystals, in
cavities of a particular kind of limestone which exists at Altyn-Tube,
a hill in the Altai Mountains, in the Urals, and in Jvorth and Central
tourmaline is a most complex substance .- almost every stone obtained
has a different composition, some varying but slightly, with mere
traces of certain constituents which other stones possess in a
perceptible degree. Consequently, it is not possible to give the
chemical formula, which might, and possibly would, be found but seldom,
even in analyses of many specimens. It will therefore be sufficient to
state the average composition, which is —ferrous oxide, manganous
oxide, potash, lime, lioracic acid, magnesia, soda, lithia, and water.
These form, roughly speaking, 25 per cent, of the bulk, the remainder
being oxide of silicon and oxide