green
light is usually followed by its complementary, red, yellow by purple,
blue by orange, green by brown, orange by grey, purple by broken green,
with all the intermediary shades of each.
Thus
when the line of sight is altered, or the stone moved, never otherwise,
the colours chase one another over the surface of the gem, and mostly
in broad splashes ; but in those gems possessing ''play of colour,"
strictly speaking, whilst the stone itself remains perfectly still, and
the sight is fixed unwaveringly upon it, the pulsations of the blood in
the eyes, with the natural movements of the eyes and eyelids, even in a
fixed, steady glance, are quite sufficient to create in the stone a
display of sparks and splashes of beautiful fiery light and colour at
every tremor.
The term " iridescence " is used "when the display of colour is keen on
the surface, rather than coming out of the stone itself. The cause of
this is a natural, or in some cases an accidental, breaking of the
surface of the stone into numerous cobweb-like cracks ; these are often
of microscopic fineness, only perceptible under moderately high powers.
Nevertheless they are quite sufficient to interfere with and refract
the light rays and to split them up prismatically. In some inferior
stones this same effect is caused or obtained by the application of a
gentle heat, immersion in chemicals, subjection to " X rays:'
and other strong electric influence, and in many other ways. As a
result, the stone is very slightly expanded, and as the molecules
separate, there appear on the surface thousands, perhaps millions, of
microscopic fissures running at all angles, so that no matter from what
position
