_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.
[Illustration: Fig. 20. Common crystal form of Llano County tourmaline.]