OPAL
For variety
and beauty of color the " precious" opal is without a peer. These
colors do not come from any chemical constituent, but are produced by
structural peculiarities which, in varying degrees, diffract the light
rays entering it and give a prismatic play of colors. It is composed of
silica with six to twelve per cent, water, and occurs in thin,
irregular veins within a matrix. Although always a soft stone, its
hardness is increased by exposure to the air. It is infusible before
the blow-pipe, but heat drives off the water and renders it opaque.
The
combinations of color are infinite. The most beautiful opals are called
" harlequin." These are distinguished by small, angular patches of
brilliant and variegated colors, which change their positions and
character as the stone is moved. Others show large blotches of color
which change as the stone is turned, but seldom show more than two
colors at the same time, one being much more pronounced than the other.
Usually the colors of the opal are broken up into small speckled
lights, which play as it is moved. The rarest form is the cat's-eye
opal, which exhibits a chatoyant line over the centre of the dome
similar to the cat's-eye and usually of a bright-green color.
Formerly
this stone was obtained almost exclusively from Hungary, the principal
mines being in the Libanka Mountain, west of Dubnik, and not far from
Czerwenitza. It is found there in the clefts and cavities of an old
lava, generally brown in color, known as andesite. It is supposed that
alkaline waters decomposed this rock and, setting free the silica,
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