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.