Tektite(Bediasite)

Fig. 17.Common crystal form of spinel.

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 approximately3 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.

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, andfig. 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.

Fig. 18.Crystal faces on topaz crystal shown inPlate V, B. This crystal habit is typical of the topaz from Mason County.

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 crystalfaces. 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.

Fig. 19.Cross section showing the proper orientation of dark-color zone in a gem cut from an irregularly colored stone.

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.

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, andfig. 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 suitablefor 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.

Fig. 20.Common crystal form of Llano County tourmaline.

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.

Anderson, B. W.(1948) Gem testing: Emerson, New York.

Baker, C. L.(1935) Metallic and non-metallic minerals and ores (precious stones),inThe geology of Texas, Vol. II, Structural and economic geology: Univ. Texas Bull. 3401, Jan. 1, 1934, pp. 568-569.

Barnes, V. E.(1940) North American tektites: Univ. Texas Pub. 3945, Dec. 1, 1939, pp. 477-582.

Dake, H. C.,Fleener, F. L., andWilson, B. H.(1938) Quartz family minerals: Whittlesey House, McGraw-Hill Book Company, Inc., New York.

Ford, W. E.(1932) A textbook of mineralogy (4th ed.): John Wiley and Sons, Inc., New York.

Kraus, E. H., andSlawson, C. B.(1947) Gems and gem materials (5th ed.): McGraw-Hill Book Company, Inc., New York.

Kunz, G. F.(1892) Gems and precious stones of North America (2d ed.): Scientific Publishing Company, New York.

Pough, F. H.(1953) A field guide to rocks and minerals: Houghton Mifflin Company, Boston.

Simpson, B. W.(1958) Gem trails of Texas: Granbury, Texas.

Sinkankas, John(1955) Gem cutting: D. Van Nostrand Company, Inc., Princeton, New Jersey.

—— (1959) Gemstones of North America: D. Van Nostrand Company, Inc., Princeton, New Jersey.

Smith, G. F. H.(1958) Gemstones (13th ed.), revised by F. C. Phillips: Methuen and Company, Ltd., London.

Sperisen, F. J.(1950) The art of the lapidary: The Bruce Publishing Company, Milwaukee, Wisconsin.

Sterrett, D. B.(1913) Gems and precious stones,inMineral resources of the United States, Calendar Year 1912, Part II, Non-metals: U. S. Geol. Survey, pp. 1023-1060.

AGem-quality celestite crystals from Travis County, Texas. Twice natural size. Lower portion of the crystals is colorless; the tips are dark blue.

BOpalized wood from the Texas Gulf Coastal Plain. Specimen at left is rich brown and tan; specimen at right is fossil palm wood and is black, reddish brown, and white. One-third natural size.

AGem-quality garnet crystals and faceted gem from Gillespie County, Texas. Natural size.

BLabradorite from Brewster County, Texas. Both stones are pale yellow. One and a half times natural size.

APink microcline crystal from Burnet County, Texas.

BSmoky quartz from Burnet County, Texas. Natural size. Colorless crystal at center back is included for color comparison.

Polished agate from gravels of the Rio Grande near Zapata, Zapata County, Texas. Bands are blue and gray; other inclusions are brown, yellow, and reddish. One and a half times natural size.

ATexas tektites (bediasites) showing variety of surface features. Natural size.

BTopaz crystal from a pegmatite dike near Streeter, Mason County, Texas. Natural size. Measurements: 1½ by 1⅝ by 3 inches; weight: 194 grams (970 carats); pale blue; mostly gem quality.

ATopaz from stream gravels near Streeter, Mason County, Texas. Natural size. Left to right: colorless worn pebble; emerald-cut pale-blue topaz, weight 10 carats; pale-blue worn pebble, weight 205 carats; step out sky-blue topaz, weight 13 carats; pale-blue worn pebble.

BTourmaline crystals in schist from Llano County, Texas.

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