light
like a doubly refracting body occurring in a " poro-dine" condition.
The most Precious, or " Noble" Opal is the best variety ; it emits rays
of light like the sun, being filled, as it were, with spangles of
divers brilliant colours. When thus richly spangled it is further named
the " Harlequin " Opal, on account of its resemblance to the motley
tints of the harlequin's dress. Then there are the so-called " common "
Opal, the " Semi-" (or half) Opal, the " Hydrophane," or Mexican Opal,
and, finally, the " Wood " Opal, or opalised wood, of which huge masses
are met with in Hungary, Tasmania, and other parts; whole trees being
found occasionally converted into the ligneous structure called " Wood
Opal." The " Hydrophane " Opal forfeits its colour and beauty when
exposed to water, then becoming translucently dull; by which cause it
happened that the stone told of by Sir Walter Scott lost its lustrous
colour when accidentally sprinkled with holy water. The Spanish Royal
Family has an unlucky opal, which has been the death of five princes,
and princesses, and is now hung round the neck of " The Virgin of
A.lmudena." In the " Harlequin " Opal the various rainbow tints are
flashed forth from small horizontal vitreous flakes, forming a kind of
polychromatic mosaic ; while in the other kinds of Opal the colours are
disposed in broad bands, or in irregular patches of comparatively large
area. The tints vary with the angle at which the light is incident, and
with the relative position between the stone, and the observer ; so
that by moving the Opal a brilliant succession of flashes can be
obtained. The colours are not due to any material pigment in the
mineral, but are optical effects,— the iridescence being a case of the
well-known colours of thin plates in layers.
