Opal is like hardened jelly or gelatin. It has no crystalline inner
structure and no crystal shape of its own—it is amorphous. This mineral
has almost the same chemical composition as quartz. Both are silicon
dioxides (silica), but opal, in addition, contains water.
Opal can be almost any color—red, yellow, blue, brown, gray, white—or it
can be colorless. It is transparent or translucent and appears glassy,
resinous, greasy, or dull. Opal has a specific gravity of 1.9 to
2.2—this mineral is a little lighter than quartz. It also is softer than
quartz. A copper penny will not scratch opal, but quartz will. Opal has
a white streak and a curved, conchoidal fracture but no cleavage.
Opal occurs in a number of places in Texas. In the Trans-Pecos country
of west Texas, it fills cracks and cavities in some of the extrusive
igneous rocks. It occurs on the High Plains of northwest Texas, and it
is found in Tertiary formations of the Gulf Coastal Plain where it
occurs as masses that fill cracks and cavities in sedimentary rocks, as
the cementing material in some sandstones (such as in the Catahoula
sandstone), and as opalized wood.
Much opal forms from underground waters that contain silicon. These
solutions move through the rocks and deposit the opal in them.
Opal is found in a number of varieties. Some show a beautiful, lustrous
play of colors that comes from inside the specimens. These varieties are
known as _precious opal_ and are prized as gemstones. In Texas, some
precious opal is found near Alpine in Brewster County. It has a milky
white to bluish-white color, is translucent, and shows a fiery orange,
red, blue, and green play of colors.
The variety known as _common opal_ shows no play of colors. It may be
white, gray, bluish, reddish, greenish, or yellowish, and it is only
slightly translucent. It is found in Brewster, Jeff Davis, Presidio, and
other counties of the Trans-Pecos country of west Texas. It occurs also
around some of the wet-weather (playa) lakes on the Texas High Plains.
In the Gulf Coastal Plain, common opal is found with chalcedony (a
variety of quartz) in Tertiary formations. A south Texas locality
sometimes visited by collectors is near Freer in Duval County.
A clear, commonly rounded, variety of opal that looks like ice is called
_hyalite_. Two areas in which it has been found are in Presidio County
in west Texas and in Llano County in central Texas.
[Illustration: Opalized wood from Washington County, Texas.]
A variety of petrified wood, called _opalized wood_, is opal that
replaced the fibers of a piece of wood. Wood opal is found at a number
of places in the Gulf Coastal Plain. It occurs there in Tertiary
formations within about 20 miles of the boundary line between areas 2
and 3 shown on the geologic map (pp. 4-5).
A soft opaline material called _diatomite_, or _diatomaceous earth_, is
made up chiefly of the skeletons of diatoms—tiny, one-celled plants that
live in fresh or salt water. These little plants are able to take silica
from the water to make opal skeletons for themselves. When the diatom
skeletons collect at the bottom of a lake or sea, they form the light,
crumbly, white, gray, or cream-colored deposit of impure opal known as
diatomite. Industry uses this material as a filter, as insulation, as an
abrasive, and as a filler.
Diatomite formed in ancient lakes on the Texas High Plains during late
Tertiary (Pliocene) and early Quaternary (Pleistocene) times. It is
found in Armstrong, Crosby, Dickens, Ector, Hartley, and Lamb counties.
Opaline Granite. _See_ Llanite.
Orthoclase. _See_ Feldspar.