_Chalcedony._—When free from impurities of various oxides and other
compounds, chalcedony has little to render it pleasing as a gemstone. It
is mostly gray, white, brown, or bluish and commonly has a waxy luster.
Some of the chalcedony found along the Rio Grande Valley and in west
Texas will take dyes, and local lapidaries have had some success in
dyeing this material various shades of blue, green, yellow, and red.
When the chalcedony is naturally colored and variegated, usually in
bands, mossy figures, or dendritic forms, it is called agate.
_Agate_ (variegated chalcedony).—The wide variety of markings and colors
available together with the ease of cutting make agate a favorite of
many lapidaries. Fine agate has been found at numerous localities in
west and south Texas. Fine plume agate, famous throughout the United
States, is found south of Alpine. Plume agate is characterized by
dendritic or tree-like inclusions and is mostly cut into very handsome
cabochons. The agate from south of Alpine commonly contains black, red,
yellow, or brown plumes within the same piece. The variety of colors and
lack of porosity of this agate make it highly desired among lapidaries.
The agate occurs loose on the surface of the ground and in the soil in
small nodules that have a very rough, brownish surface. These nodules
are mostly less than 3 inches in diameter, although specimens of gem
quality have been found that exceed 200 pounds.
Some very fine agate has been found in the vicinity of Needle Peak,
Presidio County. This material is mostly green moss agate in clear
chalcedony and commonly contains small yellow “sun-burst” figures. The
contrasting yellow and green design makes very beautiful cabochons.
Fine agate has been found south of Marfa, Presidio County. This agate is
mostly clear chalcedony with black, yellow, or variously colored plumes,
moss, or “bouquet-like” figures.
Numerous other localities in Presidio and Brewster counties have
produced good agate.
Various amounts of agate, jasper, and chalcedony occur in the gravels of
the Rio Grande in varying quantities from Big Bend National Park
downstream to Brownsville. This agate is found both in the present river
gravels and in the older river gravels that now are located on nearby
hills and slopes up to several miles north or south of the present Rio
Grande. The greatest concentration of agate and related gem materials
seems to be in the area between Laredo and Rio Grande City. Vast
quantities of excellent gem material have been removed from this area
for many years (Pl. IV). The agate occurs as rounded, stream-worn
cobbles and commonly has a thin white coating that makes it difficult to
distinguish from the abundant chert and other rocks. The agate occurs in
cobbles that are mostly 3 to 6 inches in diameter, but specimens of gem
quality that exceed twice this size are known. The agate varies greatly
in design and color. Plume, moss, banded, and sagenitic agate occur in
these gravels in a wide variety of colors. The jasper in the Rio Grande
gravels is yellow, red, green, or various shades of these and is
commonly suspended as angular fragments in clear chalcedony.
Good agate has also been found near Balmorhea in Reeves and Jeff Davis
counties and in smaller amounts at numerous other west and south Texas
localities.
_Agatized wood_ (_see_ Fossil wood, pp. 20-21).
_Carnelian_ (translucent reddish chalcedony).—This variety of chalcedony
in small quantities has been reported from near Van Horn, Hudspeth
County. Small pieces of carnelian have been found in the gravels of the
Rio Grande, but finds have been few and scattered.
_Jasper_ (impure opaque or subtranslucent quartz).—Good green, yellow,
red, and brown jasper has been found in the gravels of the Rio Grande at
all of the localities that produce agate. The colors are quite vivid,
and the material takes a fine polish. Some pieces of orbicular jasper
(jasper with circular or eye-like markings) have been found in this
material. These gravels commonly contain jasper as fragments that are
suspended in clear chalcedony; this is called brecciated jasper and
yields very handsome cabochons.
Many of the west Texas agate localities also produce jasper in quantity.
Good jasper has been reported from north of Brackettville, Kinney
County. Jasper is a minor constituent of the stream gravels in many
parts of the State.