e 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 from 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 at 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,
CuOSiO_{2} + H_{2}O, 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. This 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-Tuebe, a hill in the Altai Mountains, in the Urals, and
in North and Central America.

_The Tourmaline._

The 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,
boracic acid, magnesia, soda, lithia, and water. These form, roughly
speaking, 25 per cent. of the bulk, the remainder being oxide of silicon
and oxide of aluminium in about equal parts. It crystallises in the 2nd
(hexagonal) system, with difficult cleavage and vitreous lustre.

It will n