Gemstone of the Big Bend
| By Paul Graybeal
West Texas is famous for its wide variety of unique, beautiful agates and jaspers.
Some types are highly prized by collectors world wide. 35 ~ 60 million years ago,
volcanic activity in the Big Bend produced the environment for our beautiful
gemstones to form. Cryptocrystalline quartz crystallizing as nodules inside gas
pockets, (or in cracks to form vein type agate), can have inclusions of iron oxides
such as goethite, hematite to form delicate plumes, moss and bouquet patterns of
different colors. With no color or patterns it is chalcedony, solid colors can be jaspers,
flint, chert. Agates you can see through a cut slice, translucent or transparent, with
bands or inclusions. Agate and chalcedony is widespread throughout this volcanic
region, but only a small amount is of lapidary interest. Agate, jaspers have been found
south of Sierra Blanca, Van Horn, most of the igneous Big Bend, as well as the agate
that was carried down river to form alluvial deposits from Mexico and New Mexico.
Texas has no public lands to collect on, so there is still a lot to explore if permission
can be obtained. Most areas opened to public in the past, the surface has been
There are several types of agate besides famous red plume found in the
Cottonwood Springs Basalt. (Woodward, Walker Ranches). Black and brown plume
with banding being the most common lapidary grade. Often some black/red plumes,
surrounded in colored banding. Banded agate is common in West Texas agate, South
of Alpine it is usually white, nearly clear, but bands can be shades of red, purple or
pink, often surrounding a plume. Yellow plume can be found here, typically with much
euhedral quartz making it difficult to cut. There is orbicular agate found south of Alpine
(called peanut agate by Frank Woodward). Pat McMahan refers to pisolites/oolites in
his description. Mostly it a solid orange color inside the orbs, but this agate/jasper can
also be formed of independent orbs of different colored bands, plumes in other orbs
in the same stone. This type is often difficult to cut in cab, not all the orbs are
cemented together, and some orbs may contain small geodes. Nice rare specimens!
Most of the lapidary grade agate found South of Alpine is found in biscuit shaped
nodules with a dark red skin. Think of gas bubble in lava. Most are pea size to typical
biscuit, potato or football size becoming extremely rare. All types of agate found in
this lava flow can be found in these biscuits. About 10% of the biscuits will have a
gemstone quality to them, or only 10% of the stone will have something worth cutting.
It is a rare stone that is full red/black plume, gem grade orbicular, colored bands,
combinations within the same space, collector/museum quality.
Many of the biscuits did not fill the entire gas pocket, and had space to crystallize in
a shape with bubbles, or botryoidal crystal habit (most agate with this type has plume
under the bubbles here). Most biscuit agates here have some euhedral quartz (sugar)
inside them, and some open up to become true geodes. Flower garden agate is
formed in cracks in the host rock, making it a vein type formation. Usually orange and
red, made up of small dense microscopic plumes is found on the Walker/Woodward
border, (agate hill). I think this type should be considered a jasper as you cannot see
through it, and I have seen other locations such as in Mexico and Arizona that have
similar deposits of the orange and red flower garden patterns.
East of Alpine is the Pruitt formation, Elephant Mt., Mt. Ord, Frog Mt. Richie
Ranch, Carver agate field (Rock & Gem Augest-2017 #47) A snake like crack in the
Mts. East of Alpine are exposing some unique agate. Crosses HWY 90 to go near Ft.
Davis. Private property. I bought a lifetime collection from long time rancher, I
promised not to say where. Some nice large moss agate nodules yet to be cut, rare
metalic sagenite pseudomorphs, lots of banded agate (iris), mixes not yet described,
or named. Much large colorful nodules of moss agate like found around San Carlos,
Marfa agate was made famous by Andy Burgard in the 1940s, an article in lapidry
journal describing pastel colors in "bouquet" patterns now most sought after by savvy
agate collectors. Marfa is host to huge agate fields, found in the lava around the
Chiniti Mts are large solid white or cloud like clear agates, (chalcedony). This is the
Peatan Basalt formation.
Few agate beds seem to contain much of the colored material known as "bouquet"
or other types of cutting material. Found in isolated pockets are the gem grade
agates. Most common in lapidary grade is a black plume or black and yellow plume
agate, and white or blue fortification. Often times calcite is included. The bouquet
pattern is usually on the bottom, but can form on the entire surface and point towards
the center in small clusters. Some secondary crystallization appears as water lines or
onyx (layered agate used for cameos) inside banded hollows. A lot of this agate is still
egg shell, a thin coating of agate lining the gas pocket, hollow. Tube type agate
looking like stalactites inside some of the hollow or eggshell nodules can be found,as
well as angelwing, rare crystal forms.
Another famous location of agate in the Big Bend is Needle Peak, south of
Terlingua. Beautiful pseudomorphs after aragonite have been found here. Famous
for yellow sagenite crystals in a green moss agate called pompom or thistle agate.
Again there are many grades of agate, and some nice agate replacement in wood and
bone in the area.
Balmorhea is famous for their blue banded agate, often with black plumes. The skin
is usually black with a white patina on the surface. Some % will be iris. Onyx or water
marks can be found. It appears to be fairly wide spread in northwest of the Davis
Mountains, most seem to be naturally tumbled. The Davis mountains has little agate
I'm aware of, but does have a lot of chalcedony roses.
There are many theories about agate formation. Some suggest a silicon gel, or
silicon dissolved as a colloidal solution which saturates the host rock. Quartz is water
soluble at high temperatures, so as it cools It forms inside veins, gas pocets, or can
replace other minerals as pseudomorphs, fossils such as wood, bone and coral. No
one theory explains all types of agate.