tion, would exceed one hundred thousand pounds.

Many other stones possess one or more properties of the opal, and are
therefore considered more or less opalescent. This "play of colour" and
"opalescence," must not be confused with "change of colour." The two
first appear mostly in spots and in brilliant points or flashes of
coloured light, or "fire" as it is termed. This fire is constantly on
the move, or "playing," whereas "change of colour," though not greatly
dissimilar, is when the fire merely travels over broader surfaces, each
colour remaining constant, such as when directly moving the stone, or
turning it, when the broad mass of coloured light slowly changes,
usually to its complementary. Thus in this class of stone, subject to
"change of colour," a 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 seen 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 the stone may be
vi