The
precious opal occurs chiefly as casts of different parts of trees and
as coatings and fillings in cracks in ordinary petrified wood. The
precious opal casts rarely retain the texture or grain of the wood, but
may show such structure as bark and inclosed wood by slight color
variations or other lines of demarcation. Mr. Roop has found two very
interesting specimens of opalized cones, one measuring about 1 inch
long that has been perfectly cast by milky opal and the other, a
slightly larger cone of milky color, completely inclosed in a mass of
translucent precious opal 2\ inches thick. Twigs and limbs of
trees 2and 3 inches thick have been reproduced in precious opal of
beautiful colors. Logs several inches in diameter have been cast with
common jet-black or dark-brown opal, parts of which show fire. The
black opal gives off water and tarry matter smelling of pyroligneous
acid when heated in a closed tube. A quantity of other variously
colored common opal occurs with gem variety, as translucent purplish,
reddish-brown, gray, and white. Some precious opal has been found as
fillings in cavities in the volcanic ash. Among such specimens are
small patches of gray or white opal with a beautiful play of green,
yellow, blue, and red fire, but exceedingly brittle so that the small
pieces can be crushed between the fingers into powder. The minute
grains still display their fine color and fire after the opal has been
crushed. Opal, both in casts and in veiniets fining cracks in petrified
wood, is used for gem purposes, and some of the petrified wood
containing seams and veiniets of precious opal would yield very good
matrix opal.
The
best gem opal from Virgin Valley is unexcelled in variety and
brilliance of fire and color bv that from other localities. The cut
gems exhibit superb flashes of green, blue, yellow, and red of various
shades with milky white, gray, bluish, or brownish background which
maybe opaque, translucent, or nearly transparent. In some the color is
uniform over the whole stone or over large areas, changing, as the gem
is turned, from green to red, or from red to blue, and so on. Some of
the gems show a rich ultramarine blue in one position, and green or red
in another. Many gems display various bright colors arranged in
patches, and each patch changes color as the stone is turned. The
brilliant flashes of peacock-feather colors exhibited by the opal of
dark color yield a gem which might be called black opal, but most of is
not like the Australian gem of that name, since it occurs in thick
pieces and the colors are less localized. Most of the dark-colored
gems, no matter how beautiful in reflected light, become a rich
reddish-brown in transmitted light. The more opaque bluish-gray and
milky opal with good fire also yields especially beautiful gems. A quantity
of brittle opal which checks and cracks considerably after removal from
the mines has been found. Some of this opal has magnificent color and
fire, but close inspection shows that it contains fine cracks, some of
which are sufficiently pronounced to allow the stone to fall apart. In
many cases this tendency to check could be partly overcome by a careful
handling of the rough material, that is, by a rather slow seasoning
process in which the opal is not immediately exposed to the dry
atmosphere and considerable temperature changes of the desert, but is
kept in a moderately cool place or in moist wrapping.
