Tourmaline is a complex silicate of boron and aluminum. Other elements,
such as magnesium, sodium, lithium, calcium, iron, or fluorine, also may
be present. This mineral has a glassy to resinous luster. Only the
dark-colored varieties of tourmaline have been found in Texas. One is a
black variety called _schorl_, and another is a brown variety called
_dravite_. Other kinds of tourmaline, although not found in Texas, are
colorless or some shade of blue, yellow, red, pink, or green. Some
crystals even show more than one color.
Tourmaline is too hard to scratch with a steel file, it has a specific
gravity of 3 to 3.25, and it has a conchoidal to uneven fracture. Very
little light passes through the dark varieties, and some fragments of
schorl look like shiny, black coal.
Tourmaline occurs as masses without crystal shapes, but crystals are
commonly found. The crystals are prism-shaped and have small vertical
grooves, called _striations_, on the prism faces. When you look at some
crystals from an end, you will see that the cross section is a triangle
with the sides bowed outward.
[Illustration: Black tourmaline crystals with milky quartz from north of
Llano, Llano County, Texas.]
Both the black and the brown varieties of tourmaline have been found at
several places in the Llano uplift of central Texas. One well-known
locality is at Town Mountain north of Llano in Llano County. Here, the
tourmaline occurs in milky quartz that is associated with Precambrian
granite rocks. In west Texas, in Culberson and Hudspeth counties, black
tourmaline occurs in pegmatite rocks in the Van Horn Mountains, the
Carrizo Mountains, and the Wylie Mountains. In the Eagle Mountains of
Hudspeth County, it is found in metamorphic rocks as well as in
pegmatites.
Some tourmaline formed from hot fluids containing boron that were given
off by magmas far below the earth’s surface. These fluids traveled up
through cracks and other openings in overlying rocks. As the fluids
reacted with other elements and compounds, the tourmaline formed.
The clear, light-colored varieties of tourmaline are much admired, and
they are more widely used as gemstones than are the dark-colored
varieties. Some collectors, however, find that the dark-colored Texas
tourmalines, when cut and polished, make shiny, attractive gemstones.
Some tourmaline is used as grinding material, but no Texas tourmaline is
produced for this purpose.
Travertine. _See_ Calcite.