Llanite is a unique rock that is found only in Llano County in central
Texas. This intrusive igneous rock is made up of easily seen crystals
and grains of quartz and feldspar that are scattered through a
brown-colored mass of extremely small mineral grains. The quartz is
beautiful, sky-blue, and opal-like; the feldspar has a rusty pink color.
(Because the quartz looks like opal, this rock often is called _opaline
granite_.) The mineral grains that make up the brown-colored mass are so
tiny that they can be identified only with a microscope. They are
quartz, feldspar, mica, fluorite, and apatite.
Llanite formed during Precambrian time. Molten rock material forced its
way upward into cracks that cut across granite and schist rocks while
the rocks were still far underground. This hot magma remained in the
cracks where it cooled and hardened to form long, narrow, wall-like
masses (called _dikes_) of llanite. We can see some of the llanite dikes
exposed at the earth’s surface to the north and northeast of Llano in
Llano County because the overlying rocks have weathered away.
Llanite has been quarried from one of the dikes west of Babyhead in
northern Llano County. Because llanite is both attractive and strong, it
has been used as an ornamental stone and as a monument stone.