LXXII.
OPAL.
The gem
is a hydrated silicate, insoluble in acids, and when subjected to a
strong heat it becomes white, leaving a remarkable quantity of water.
It
possesses the property of reflecting all the coloured rays of the
prism, and this property originated its name, derived from the Greek υπ, root of opτω, (obsolete), to see, and άλλοs, other, in allusion to the multiplicity of its colours.
The changing lights which it emits are caused by an immense quantity of minute fissures.
In
general it is injured by potash. It shows no trace of crystallization,
or of double refraction. Its light is almost always resinous. It is
found in scattered pebbles and in veins -within a matrix of a reddish
tint spotted with white, in trachytic districts.
Its
hardness varies, although it always yields to the file ; it is of very
fine texture, and takes a beautiful polish. Very brittle, its breakage
is bright and perfectly concave ; it does not melt under the action of
the blow-pipe, but crackles, splits and loses colour.
"When
just extracted from the damp earth, the opal is very soft and destitute
of light, but the air and sun soon succeed in giving it various degrees
of beauty and consistency ; and it is a very curious phenomenon to see
it sensibly hardening and the colours being created as the water
evaporates from the minute
