Quartz, silicon dioxide, is one of the most common minerals. It is
glassy, waxy, greasy, or dull and is transparent or translucent. Pure
quartz is colorless, but impurities make some varieties white, black, or
a shade of red, yellow, blue, violet, or brown. Quartz is a hard
mineral. It scratches window glass and cannot be scratched by a pocket
knife or even by a steel file. It has a specific gravity of 2.65. The
curved, conchoidal fracture shown by many specimens helps identify it.
Quartz is plentiful in Texas. It occurs in igneous rocks, such as
granite, llanite, and pegmatite; in metamorphic rocks, such as
quartzite, schist, and gneiss; and in sedimentary rocks, such as some
sandstone, conglomerate, and breccia.
[Illustration: Quartz crystal, with inclusions, from Burnet County,
Texas.]
Quartz is found as crystals and as masses. Some of the masses are
coarsely crystalline, but some are made up of extremely small
crystalline particles called _cryptocrystalline_ quartz. Some of the
cryptocrystalline varieties of quartz found in Texas are chalcedony,
chert, and jasper. Some of the coarsely crystalline varieties found here
are amethyst, milky quartz, rose quartz, smoky quartz, and rock crystal.
[Illustration: Amethyst geode from the Alpine area of Brewster County,
Texas.]
A colorless, glassy variety of quartz, called _rock crystal_, is clear
enough to see through. It is found as crystals that are 6-sided prisms
with pyramid-like faces on the ends. This variety is commonly associated
with igneous rocks, such as those of the Llano uplift area of central
Texas and of the Trans-Pecos country of west Texas. It is commonly used
as a gemstone and is made into necklaces, earrings, and other jewelry.
Some specimens of rock crystal have slender, needle-like crystals of
other minerals, such as tourmaline, actinolite, or rutile, enclosed in
them.
A clear, glassy variety of quartz, _amethyst_, has a purple or violet
color. It, like rock crystal, is commonly found in 6-sided prisms with
pyramid-shaped ends and is also prized as a gemstone. Amethyst has been
found in Precambrian rocks in the Llano uplift area of central Texas.
(Amethyst Hill, a locality well known to collectors for many years, is
in northeastern Gillespie County.) In west Texas, amethyst has been
found in Cenozoic igneous rocks in the Sierra Blanca and Quitman
Mountains of Hudspeth County and in the Alpine area of Brewster County.
[Illustration: Milky quartz from Burnet County, Texas.]
A variety of quartz with a milk-white color and a glassy to greasy
luster is called _milky quartz_. It occurs either as crystals or as
crystalline masses. Very little light will pass through it. In central
Texas, milky quartz occurs abundantly in the Precambrian rocks of the
Llano uplift area in Blanco, Burnet, Gillespie, Llano, and Mason
counties. It also is found in some of the rocks of the Trans-Pecos
country of west Texas, such as in the Carrizo Mountains of Culberson and
Hudspeth counties. Other good places to look for this variety of quartz
are in the sands and gravels along many streams in Texas.
Some quartz has a glassy to a greasy luster and a rose or pink color.
_Rose quartz_, as this variety is called, commonly occurs as masses
rather than as individual crystals. It can be found along some of the
streams in Texas and also in igneous rocks, such as those of the Llano
uplift area of central Texas.
A kind of quartz with a smoky brown, a smoky yellow, or a dark
brownish-black color is called _smoky quartz_. Its luster is glassy, and
it may be either translucent or transparent. Smoky quartz is commonly
found as crystals that are shaped like 6-sided prisms with pyramid-like
ends. It is commonly associated with igneous rocks, and beautiful
specimens have been found in the Lake Buchanan area of Llano and Burnet
counties in central Texas.
[Illustration: Smoky-quartz crystals from Burnet County, Texas.]
A cryptocrystalline variety of quartz, _chalcedony_, has a waxy to dull
luster and a tan, white, gray, or light-blue color. It is translucent
but not transparent. Chalcedony does not have its own crystal shape but
instead is found in masses that line or fill cracks, pores, and other
cavities in rocks. It is formed when water containing silicon slowly
seeps into these openings in the rocks and deposits the silicon dioxide
there as chalcedony.
Chalcedony commonly occurs in some of the Tertiary rocks of the Gulf
Coastal Plain. For example, chalcedony associated with opal is found
near Freer in northern Duval County. In the High Plains of west Texas,
it is found in alkali-lake deposits, such as at Shafter Lake in Andrews
County and at Cedar Lake in Gaines County. In the Trans-Pecos country of
west Texas, it can be found filling small cavities in extrusive igneous
rocks.
[Illustration: Polished agate from Rio Grande gravels of Zapata County,
Texas.]
A variety of chalcedony that generally is made up of more than one color
is called _agate_ (although agates consisting of several shades of a
single color are also found). The colors may be spread out unevenly so
that the agate has a cloudy appearance, or they can be arranged in wavy,
in straight, or in concentric lines or bands. If the bands are straight
and parallel, the specimen is called _onyx_. Agate that has a moss-like
or tree-like design in it is called _moss agate_. Some agates make
attractive gemstones when cut and polished.
[Illustration: Jasper from Uvalde County, Texas. Dark areas are brownish
red; light areas are a yellowish-tan.]
Much agate has been found filling cavities in Cenozoic igneous rocks in
Brewster, Presidio, and other counties in the Trans-Pecos country of
west Texas. It has been found also in an area about 10 to 15 miles wide
along the Rio Grande, mostly in southern Webb County and in Zapata and
Starr counties.
Trees and other plants have been replaced by agate. Many specimens of
_agatized wood_ have been collected from Tertiary formations in Fayette,
Gonzales, Lee, Washington, and other counties of the Gulf Coastal Plain.
(The agatized wood, along with opalized wood, occurs within about 20
miles of the boundary between no. 2 and no. 3 on the geologic map, pp.
4-5.)
A hard, compact, slightly translucent variety of cryptocrystalline
quartz is called _jasper_. It commonly has a red, brown, or yellow color
due to the presence of an iron oxide, such as hematite. Some jasper is
made up of irregular bands of more than one of these colors. This
variety of quartz often is polished to make attractive gem or ornamental
stones. It has been collected at several localities in Texas,
particularly from creek and river gravels. Starr and other nearby
counties along the Rio Grande have furnished a number of good specimens.
A hard, smooth, compact, translucent rock that is made up mostly of
cryptocrystalline quartz is called _chert_ or _flint_. It is white,
black, or some shade of gray, brown, or pink, and its luster is waxy,
slightly glassy, or dull. Chert is found in many creek and river gravels
in Texas. It also occurs with limestone, such as in the Lower Cretaceous
Edwards Limestone of central Texas and in the Ordovician Ellenburger
strata in the Llano uplift area. Chert also is found with the Ordovician
rocks of the Marathon area of Brewster County.
Geologists do not agree on whether chert and flint are two names for one
variety of rock, or whether each is a separate variety. Some, however,
now give _chert_ a geological meaning and _flint_ an archaeological
meaning. They use the word _chert_ to describe geological formations or
rock specimens. They give the name _flint_ to the same rock when it has
been used by Indians in making arrowheads, scribers, scrapers, and
spearheads.