which
we call opalescence. It has sometimes a glassy, but often a waxy,
luster, the latter when pronounced giving rise to the varieties known
as wax opal and resin opal. When opal has the banded structure of agate
it is known as opal-agate; when it has the color of jasper, as
jasper-opal; and when that of chrysoprase, as prase-opal. But none of
these varieties is used in any quantity as gems. This distinction is
reserved almost wholly for the variety known as noble or precious opal.
This is opal which exhibits a play of colors. No essential chemical or
physical distinction between noble opal and other varieties is known.
In a large vein of opal portions will exhibit the play of colors and
the remainder will not; but why the difference has not yet been
determined. The uncolored opal is known by the Australian miners as
"potch," while that which is precious is known as "colors." The origin
of the varied coloring, i. e., the iridescence, is not positively
known. Some regard it as due to interspersed layers containing
different percentages of water, which break up the rays of light
somewhat as a prism does, while others think that minute cracks and
fissures through the stone furnish surfaces from which the rays are
reflected in different colors back to the eye. Some opals which are
dull and lusterless when dry, exhibit considerable play of color when
immersed in water, and this fact seems to favor the first theory of the
cause of the iridescence, but the subject is not understood. The
character of the play of colors differs in different opals, and this
gives rise to different varieties. The true noble opal has the color
quite uni. formly distributed. When the color appears in flashes
chiefly of red and yellow, the stone is known as fire opal; of blue, as
girasol; and chiefly of yellow, as golden opal. When the patches of
color are small, angular, and uniformly distributed the stone is called
harlequin opal, and if these are long and somewhat parallel, flame
opal. These colors are not, of course, inherent in the stone, its color
varying from colorless to opaque-white. The black opals sometimes seen
are usually of artificial origin, being made by soaking ordinary opals
in oil and then burning the oil. The brilliancy of the stone is thus
increased; but it is made fragile and liable to lose color. Any opal,
however, may lose its play of colors on being heated too highly. It is
the variety and brilliancy of the changing colors which give to opal
nearly all its desirability as a precious stone, for the qualities of
hardness, transparency, and rich body color, which give to most other
gems their value, are lacking in it. But, together with the beauty of
its changing colors, opal possesses an advantage over all other gems in
that it cannot be successfully imitated. It is said that the Romans
were able to
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