Part 4

Feldspar quarries in large pegmatites in northeastern Gillespie County have yielded attractive quartz crystals, some of which contain smoky phantom crystals and tourmaline inclusions.

Some pieces of rock crystal enclosing green, needle-like actinolite crystals have been found near the Llano-Gillespie-Blanco County corner. This material is not suitable for faceted gems but does lend itself to interesting and attractive cabochons.

Colorless quartz crystals commonly are found lining small chalcedony geodes in Brewster, Presidio, Culberson, Hudspeth, Reeves, and Jeff Davis counties. These crystals are most commonly less than 1 inch long but are mostly very clear.

Rock crystal has been found in crevices of petrified wood in many east and southeast Texas counties, although the crystals are mostly quite small.

Many lesser occurrences of rock crystal, too numerous to mention, are located within the State.

_Rose quartz_ (pink quartz).—Rose quartz occurs at various localities in Burnet, Llano, Mason, and Gillespie counties, but the amount of material is mostly small and the greater part unsuitable for gem purposes. Some good pink rose quartz occurs near Town Mountain, Llano County, but this material does not have flawless areas large enough to yield faceted stones of more than a few carats. Rose quartz is always slightly milky, or cloudy, and does not cut into brilliant faceted stones. The Town Mountain rose quartz has been cut into attractive cabochons.

_Smoky quartz_ (brown, yellow-brown, and golden-brown quartz).—Several Texas localities have produced fine smoky quartz. Baringer Hill, a noted rare-earth minerals pegmatite locality in Llano County, contained some smoky quartz crystals that were estimated to weigh over 1,000 pounds, and the locality produced many smaller crystals that were of gem quality. Baringer Hill was flooded by the completion of Buchanan Dam in 1938 and is presently under the waters of Lake Buchanan. A few fine golden-brown gem-quality crystals have been found along the lake shore and in small pegmatites nearby (Pl. III, B.).

Feldspar quarries in northeastern Gillespie County have produced smoky quartz crystals that exceed 1 foot in length, but these crystals are mostly flawed, possibly as a result of blasting, and mostly contain only small clear areas.

Good color smoky quartz crystals are found with topaz in the pegmatites and stream beds in Mason County, near Streeter, Grit, and Katemcy. These crystals tend to be lighter colored than those near Lake Buchanan, but they commonly contain large flawless areas.

CRYPTOCRYSTALLINE VARIETIES

_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.

Sanidine

_Composition_: KAlSi₃O₈; commonly contains some sodium. _Crystal system_: monoclinic. _Hardness_: 6. _Specific gravity_: 2.57 to 2.58. _Luster_: vitreous to pearly. _Color_: colorless, white, pale yellow, and gray. _Streak_: uncolored. _Cleavage_: three directions. _Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent to subtranslucent. _Refractive index_: 1.52 to 1.53.

Some feldspars, including sanidine, show a nice blue sheen in reflected light parallel to certain crystallographic directions. Stones having this property are called moonstone. A clear yellowish sanidine showing an attractive blue sheen has been found in Brewster, Jeff Davis, and Presidio counties. The individual pieces are small, the average size being about one-eighth inch. The sanidine is found loose in the soil at some localities where it has weathered out of rhyolite, and specimens of the sanidine in the parent rock are not difficult to obtain. Very small cabochons can be cut from this material, but few lapidaries have done so because inexpensive larger pieces of moonstone can be obtained easily from foreign sources. However, the west Texas sanidine does show a blue sheen when cut and polished.

Spinel

_Composition_: MgAl₂O₄ (magnesium may be replaced in part by ferrous iron or manganese and the aluminum by ferric iron and chromium). _Crystal system_: isometric. _Hardness_: 8. _Specific gravity_: 3.5 to 4.1. _Luster_: vitreous to sub-metallic. _Color_: black, pink, red, blue, green, yellow, brown, and violet. _Streak_: white. _Cleavage_: one direction, imperfect. _Fracture_: conchoidal. _Tenacity_: brittle. _Diaphaneity_: transparent to opaque. _Refractive index_: variable, approximately 1.72 to 2.00.

In many areas of the world, fine quality, beautifully colored, transparent spinels are found and used as gems. The only gem-quality spinel reported thus far in Texas is black and opaque. Near Eagle Flat in Hudspeth County, black spinel crystals have been found associated with augite and natural glass; these minerals are weathering out of an intrusive igneous rock. The spinel crystals have an octahedral form which is common for this mineral (fig. 17). Most of the spinels are free of flaws, but because of their black color they have little value as gems. The crystals are found loose in the sand of streams near the outcrops of the igneous rock or embedded in the rock. They seldom exceed half an inch in diameter. These stones are primarily sought by collectors.

Tektite (Bediasite)

_Composition_: A natural glass, approximately 75% SiO₂, 15% Al₂O₃, 4% FeO, also MgO, Na₂O, K₂O, and traces of other elements. _Crystal structure_: amorphous. _Hardness_: 5 to 6. _Specific gravity_: 2.33 to 2.44. _Luster_: vitreous, often dull on weathered surfaces. _Color_: dark brown, greenish brown, appears black in thick sections. _Streak_: uncolored. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle. _Diaphaneity_: transparent to subtransparent. _Refractive index_: 1.488 to 1.512.

The average bediasite size is about 1 inch in diameter, although specimens approximately 3 inches in diameter are known. The uncut tektites are very interesting, showing a variety of shapes and surface features (Pl. V, A) and many exhibit contorted flow structure. The surface of many tektites is grooved or furrowed, while on others it is smooth or frosted. The Texas tektites are known as “bediasites,” after place names in Grimes County traceable to the Bedias Indians who formerly lived there.

Dark brown and greenish-brown tektites have been found in Texas in gravels at scattered localities in Walker, Grimes, Brazos, Burleson, Lee, Fayette, Gonzales, Lavaca, and DeWitt counties. Outside of Texas the only other authenticated tektite localities in the United States at the present time are in Dodge and Irwin counties, Georgia. A fragment of a similar tektite has recently been reported from near Martha’s Vineyard, Massachusetts. The tektites reported from Oklahoma are now known to be pebbles of obsidian.

Although tektites have little value or beauty as gemstones, they have been cut by lapidaries as both faceted and cabochon stones. Tektites take a high polish but are mostly so dark in color that they appear black.

The origin of tektites is of great scientific interest and is currently the subject of much debate. Some scientists believe that tektites are of meteoritic origin, while others believe that tektites were formed by various terrestrial processes. Since no one has actually observed a tektite to fall or form, and many of the theories of origin are difficult to prove without direct observation, the origin of tektites is likely to remain in controversy for some time.

Topaz

_Composition_: Al₂(F, OH)₂SiO₄. _Crystal system_: orthorhombic. _Hardness_: 8. _Specific gravity_: 3.4 to 3.6. _Luster_: vitreous. _Color_: pale blue, sky blue, greenish, white, wine yellow, straw yellow, grayish, pink, reddish, and orange. _Streak_: uncolored. _Cleavage_: one direction, basal, highly perfect. _Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent to subtranslucent. _Refractive index_: about 1.60 to 1.63. _Dispersion_: moderate.

Various yellow and smoky colored quartz gems are offered for sale as “Spanish Topaz,” “Smoky Topaz,” “Madeira Topaz,” and “Topaz Quartz.” These names are entirely misleading and should be dropped from usage.

Fine gem-quality white, pale-blue, and sky-blue topaz has been found near Streeter, Grit, and Katemcy, Mason County. This Texas gem material compares favorably in color, size, and clarity with topaz found anywhere in the United States. Fine crystals of topaz (Pl. V, B, and fig. 18) occasionally are found in pegmatite dikes associated with quartz, black tourmaline, cassiterite, and pink microcline. Many of the gem-bearing pegmatites have been eroded away, leaving the topaz concentrated in the stream beds. The stones mostly occur as frosted, stream-worn pebbles (Pl. VI, A) in the numerous small creeks in the area. The topaz is heavier than the quartz and microcline that compose the stream gravel and is commonly found immediately on top of the granite bed-rock in the bottom of the stream bed. The stones tend to lodge behind boulders or small dikes cutting across the stream.

The white or colorless stones are by far the most common, outnumbering the bluish stones about ten to one. The color of the blue stones tends to be irregularly distributed in zones parallel to the crystal faces. Topaz that is colored in this manner should be cut with the best blue color near the bottom or culet of the gem (fig. 19). If done correctly, this will give the entire gemstone the desirable blue color.

COLORLESS BLUE

The colorless stones can be turned pale yellow, yellowish brown, or straw yellow by exposure to X-ray radiation, and some of the bluish stones will fluoresce faintly yellowish under ultra-violet light.

The largest gem-quality topaz crystal yet found in North America has come from Mason County. It is a pale-blue crystal weighing 1,296 grams, now in the collection of the U.S. National Museum. Several other large pieces, some weighing over a pound, have been found. One large crystal, exact weight unknown, was found near Katemcy. Several gem cutters have estimated that this stone could easily yield a single, flawless pale-blue gem of about 500 carats. Many large gems have been cut from topaz found in this area, including at least one stone of over 300 carats.

One obstacle in the cutting of topaz is its perfect basal cleavage. The gemstone should be oriented so that no facet of the stone will be parallel to or within less than about 5 degrees of the cleavage direction, or the facet may be very difficult or impossible to polish.

It is difficult to estimate the productivity of this area since its discovery in the early 1900’s. Few systematic attempts have been made to exploit the deposits, and a great amount of the topaz thus far recovered has been found by private collectors. The Mason County topaz deposits are still very productive, and additional exploration may uncover even more gem-producing areas.

Topaz has also been found in stream gravels or pegmatites in Burnet, Llano, Gillespie, and El Paso counties but very rarely in gem quality.

Tourmaline

_Composition_: H₉Al₃(B·OH)₂Si₄O₁₉; hydrogen often replaced by iron, magnesium, calcium, or fluorine. _Crystal system_: hexagonal. _Hardness_: 7 to 7.5. _Specific gravity_: 2.98 to 3.20. _Luster_: vitreous to resinous. _Color_: black, brownish black, brown, blue, green, red, pink, yellow, and gray. _Streak_: uncolored. _Cleavage_: two directions, very imperfect. _Fracture_: subconchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent to opaque. _Refractive index_: about 1.62 to 1.64.

Black tourmaline is schorl; brown tourmaline, dravite.

Good crystals of black and dark brown tourmaline occur at Town Mountain near Llano, Llano County. The tourmaline crystals average about 1 inch in length, do not commonly exceed 2 inches, and are associated with white vein quartz. The quartz completely encloses the tourmaline, but the crystals can be broken free or the quartz can be trimmed away with the use of a diamond saw. The latter procedure is recommended whenever possible, for it is very easy to shatter the tourmaline crystals while trying to remove them from the quartz by other means. Many of the crystals are completely unsuitable for cutting, being too brittle or too badly cracked and flawed. However, some small crystals have been found that are of sufficient quality and size to yield flawless stones of a few carats. Few of these stones have been cut since the tourmaline is so dark that it appears opaque, and few persons find a gem of this nature attractive.

Good black and dark brown crystals of tourmaline associated with andalusite and graphite occur in the Packsaddle schist (Precambrian) near Sunrise Beach, Llano County (Pl. VI, B, and fig. 20). Although generally smaller in diameter than the crystals found at Town Mountain, they commonly exceed 3 inches in length, although the average size is a little over 1 inch. Many of these crystals are suitable for cutting into opaque or nearly opaque stones of about 5 or 6 carats.

Black tourmaline has also been found in Hudspeth and Culberson counties but not of sufficient quality to be used as a gemstone.

Turquoise

_Composition_: hydrous phosphate of aluminum and copper. _Crystal system_: triclinic. _Hardness_: 5 to 6. _Specific gravity_: variable, 2.6 to about 2.8. _Luster_: dull, sometimes waxy. _Color_: sky blue to greenish blue. _Streak_: white to greenish. _Cleavage_: none in massive material, two directions in crystals. _Fracture_: conchoidal to subconchoidal. _Tenacity_: brittle. _Diaphaneity_: subtranslucent to opaque. _Refractive index_: 1.61 to 1.65.

Turquoise of good sky-blue to greenish-blue color has been found a few miles southwest of Van Horn, Culberson County. Several shallow pits were dug at this locality about 1910; however, the amount of turquoise produced was small. The main occurrence of the turquoise was in seams about 1 millimeter thick along joints in the fine-grained rocks of this area. Persons who have visited Culberson County more recently report that even minute traces of the turquoise are now difficult to find at the old prospect pits. However, further prospecting in the area might yield some additional localities.

Small amounts of turquoise have been reported near El Paso, El Paso County, and also in volcanic rocks near the Jeff Davis-Brewster County line, north of Alpine.

A small amount of turquoise has been mined from several localities a few miles northwest of Sierra Blanca in the Sierra Blanca Mountains of Hudspeth County.

GLOSSARY