OPAL.
NEVADA.
A
quantity of magnificent precious opal was mined during 1912 in Humboldt
County, Nev. Two groups of claims have been located about 9 miles apart
in Virgin Valley, a tributary of Thousand Creek. One group of claims
has been developed by Ivan Dow, of Nevada City, Cal., and the gem
material is handled by the International Gem Co., of New York City. The
other deposit was located by J. F. Heeney and Deb Roop, of Reno, Nev.,
during 1912. Some beautiful gems were obtained during prospecting at
this locality. A comparison of a few specimens from this deposit
supplied by Messrs. Heeney and Hoop, with a quantity in the office of
the International Gem Co., shows that the type of gem and its
occurrence are simfiar at the two localities.
The
region has been described by J. C. Merriam ' as composed of rocks of
Tertiary age belonging to the Miocene. These rocks exposed in Virgin
Valley consist in large part of volcanic ash and tuff, some of which
have been deposited in shallow shifting lakes. The ash is somewhat
indurated and has undergone considerable alteration, in the
opal-bearing parts, through the agency of solutions, possibly those
from which the opal was deposited. The locality has also proved of
geologic interest for the mammalian fossils it contains. At certain
horizons of the rock formations large logs of beautifully petrified
wood are abundant and in one bed stems and leaves have accumulated in
sufficient quantity to form a thin deposit of lignite. The opal occurs
at the horizon carrying petrified wood, with which much of it is
associated.
Of
the many specimens of precious opal examined the majority were portions
of petrified limbs or twigs of trees in which the opal appears to be" a
cast rather than a replacement of the body and texture of the wood. In
some specimens there has been a partial replacement of the wood by gem
opal, which therefore retains the texture and grain of the wood.
Precious opal also occurs as a filling in cracks and cavities both in
petrified wood and in the rock mass itself. The ordinary petrified wood
examined consists of common opal and chalcedony. Limbs of trees up to
2 and 3 inches thick are reproduced in precious opal of gorgeous color
or consist in part of common brown or black opal grading into gem opal.
A piece of jet black common opal when heated in a closed tube gave off
water and a strong tarlike odor showing the presence of inclusions of
organic matter to which the color is doubtless due. A quantity of other
variously colored common opal occurs with the gem variety, as
translucent purplish, reddish-brown, gray, and white. A peculiar
specimen consisted of volcanic ash through which was scattered a large
number of small patches of opal. This opal has a magnificent play of
green, yellow, blue, and red (or fire), but is exceedingly brittle, so
that the small pieces can be crushed between the fingers into powder.
The minute grains display their fine color and fire after the opal has
been crushed. Specimens of charred wood also occur in the opal-bearing
ash.
The
best gem opal from this region is unexcelled in variety and brilliance
of fire and color by that from other localities. The cut
1 Science, new ser., vol. 26,1907, pp. 380-382.
