LI.
IRIS.
A variety of
very transparent and clear rock-crystal, remarkable for its peculiar
property of reflecting all the prismatic colours, by means of certain
natural crevices which it contains.
There
is artificial iris, which is formed by producing chinks in the
rock-crystal. And this can be done by the blow of a mallet ; by
throwing the crystal into boiling water ; or, finally, by first warming
it and then throwing it into cold water.
But
by all these methods crevices are produced which take their rise at
external points, and never reach the centre, which remains like an
intact fruit stone ; whereas the natural iris, which alone is valuable,
has chinks which part from the centre, and do not reach to the edges,
which remain smooth and compact.
The
most beautiful iris, called Oriental, has very vivid colours, but these
are rendered less clear by a slight bluish-milky tint. Its specific
gravity is equal to 2-640.
The iris called " calcedonyosa " presents but three visible colours when looked at against the sun.
That which is called " citrina " is a false topaz, but of very hard texture.
Many gems possess this particular phenomenon of the iris ; but this, instead of increasing, diminishes
