|
PRECIOUS STONES. 185
first
mineralogical variety. Those kinds which have a well-marked crystalline
structure but are of dull tint and not transparent are classed as the
second variety, common Corundum, the Adamantine Spar of Black, and the
Adamas Siderites of Pliny. The more common colours seen are dull blue
to grey and smoke-brown to black. The third variety does not depend so
much on colour as on its less defined crystalline structure. It is
called Emery, and is the substance so well known as a polishing agent;
but even in "knife-powder" one may sometimes isolate quite distinct
though microscopic Sapphires. Emery is usually granular and massive.
The
lustre of the purer varieties is adamantine, but it passes gradually
into vitreous in the less pure forms. On the basal plane it is
sometimes pearly.
Diaphaneity. As indicated above, the purer forms are transparent, others sub-transparent to opaque.
It
is doubly refracting, but not in a marked degree, the index for yellow
light being in the ordinary ray 1.769, and in the extraordinary ray
1.760, and the dispersion is slight. Being doubly refracting, it is of
course capable of lighting the field under the crossed Nicols of the
polariscope. The coloured varieties show a marked dichroism, especially
in specimens having a deep colour, in which the phenomenon may be so
well marked as to be obvious to the eye. A Ruby, when seen through the
dichroscope, shows one image of a rich red inclining to violet, while
the other is of a paler red. Sapphire shows images of a rich deep blue
and a pale greenish blue; Oriental Amethyst shows a rich violet image,
and a very pale violet or colourless one; Oriental Emerald shows a blue
and a green image.
|
||
|
|
||