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